Hey i just now started to have an idea about a noble man, that just arrived to the continent to start and build his fortune/base. 

and i was thinking, does it exist finished home bases for dnd campaigns. 

like maybe you have a big land, and you could chose to build farmland and this will cost you 500 gold.

and if you would like to build castle walls around your land, it will cost you 10 000 gold. 

could you guide me to links towards stuff like this.

a finished drawn map, where you could add parts when you upgrade them. And they are not visible when you don’t have them?

finished map to lay on the table, or a program on a computer. I’m just wondering what options are out there  

I know some people like Strongholds and Followers.  Here is a detailed review.

https://justicearman.com/2019/07/14/my-thoughts-on-mcdms-strongholds-and-followers-part-i/

This would be my suggestion as well (Strongholds and Followers).

Kingdom's & Warefare (also by MCDM) felt more immediately useful for me on this quest. I also use the 2nd edition Birthright Campaign Setting rules - which helps to manage actual kingdoms, trade routes, etc.

There are also some words on the cost to build structures in the DMG.

I would love WotC to make bases and whatnot the basis for a future sourcebook.

As a huge fan of Birthright in general it always warms my heart to see it get a mention.  I thought the rules it presented were nicely done, but they are definitely based around the idea you will be running a campaign with multiple regents (of lands, temples, guilds, or magical sources) actively moving in the world.  Picking up the setting + the domain management rules is inexpensive on DMs Guild and there are 5E conversions for it you can find with a simple search.

With that said, if the OP is looking more for simply what a character might do and spend money on if they become the ruler of a region, something from Matt Colville recommended above is probably more targeted.

I tend to use a basis of an underlying Feudal structure in most of my settings (not always), which means that by default the State owns everything and so there needs to be a reason to allow them to do so, or they will come into conflict with the ruling Authority system.

In less odd terms: they need the permission of the King to do that.

Getting it is a major roleplay thing, but one of the key elements there is renown -- have they earned that? If they have, then they are given a set of basic rules, have to swear fealty (a complex concept in and of itself that does include a magical oath) and then they have to get busy.

I have also been playing a really long time, so I use modified version of the 1e AD&D construction rules, but given how many chapbooks were produced for 3.5, I wouldn't be surprised is there were a few of them and things like mentioned above.

To me, the Coleville books, are good for folks for whom 5E is their first time around the block or were more casual players of prior editions (so likely didn't do the stronghold thing).  They do serve an important service to 5e in a "huh, this edition missed this (maybe again in the edition cycle), so here you go" tool building.  

The way I tend to handle players going, I guess not to the next level so let's call it putting down a foundation in their world, is sure set some sort of material cost for building a structure or repurposing/rehabilitating an existing structure, but really since money and riches aren't really "alive" in 5e I'd focus less on the leveling up of funds, and just presume at certain point of adventuring the characters are just "well off" having socked away currency or put their money into actual money making ventures while off adventuring etc.  So the "game" of building a stronghold becomes less one about funds but about the actual building and let's call it the environmental impact of the building.  Anyone who's ever tried to build or even renovate in a large metro area, and even many suburbs, can see how this plays out.  

What literally entitles the character to seize or make claim to ground to build a stronghold?  Dungeons and Dragons played as "Claims and Deeds" can be fun.  Do you really presume there are "unclaimed" lands in your world (not going to lean too hard on colonialist assumptions, but that could influence this).  Before a character can put their literal stake in the ground, determine, if your world building hasn't done so already, what sovereigns and power grant title and deed over the land.  If the characters are going to try to go "off grid" or "off hex" to avoid potential tax or tithe or court obligations to that power, there may still be a power structure out in "the wild," the characters just may not know about it until ground is broken and another nation, or entity or the earth itself responds to the characters' presumption that the land was just "for the taking."  In either case, maybe there isn't a current "power" directly overseeing the area, but in time as you build infrastructure, people with rights to the ground may come forth and you may have to contend with civil and maybe combative disputes.

Assuming the characters aren't killed or run off as insurgents or upstarts and do negotiate rights to their land, there are still complications to building.  Materials have to be sourced, that leaves the venture open to a range of theft, sabotage, and piracy.  If the characters are copper pinchers, they may try to work out contracts with trades folks willing to undercut the guild standards, that may put your construction at a site of strife.  What's the remedy for the trades folk if during construction they suffer supernatural injury inflicted by one of your old enemies, now that you've put a flag in the ground to tell everyone where you're at?

That last point goes into the stronghold's role in your campaign.  The stronghold while maybe considered a "safe place" for the PC, or that was the intent, in game terms it's really a device to further challenge the players, maybe even pin them to a degree.  There's adage that home ownership is a full time job, and I suppose that's why stronghold owners tend to have major domo types.  But the stronghold becomes the de facto space for all your past, present and future enemies to come to in order to air whatever grievance they may have with the stronghold's owner.  Moreover, remember when the PCs were lower level and came up with any number of justifications to break into a wizards' tower, or a old warrior's armory, or a thieves' guild's vaults, or a Cleric's Temple, etc and take their stuff?  Your PCs are now on the receiving end of that ... how do you play it out?

tl:dr don't worry about counting coin so much as what role will establishing the stronghold have on your game world.  A stronghold or base or what have you is literally the site where PCs can start to really have an impact on the game's world building.  

Strongholds and followers is good , Acquisitions Inc has a different take that has some good stuff as well. As Dorsey says 1e had some decent rules because it was expected that you would build a stronghold somewhere between L10 & L15.

Quote from sneaky_leaf >>

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like maybe you have a big land, and you could chose to build farmland and this will cost you 500 gold.

and if you would like to build castle walls around your land, it will cost you 10 000 gold. 

could you guide me to links towards stuff like this.

a finished drawn map, where you could add parts when you upgrade them. And they are not visible when you don’t have them?

finished map to lay on the table, or a program on a computer. I’m just wondering what options are out there  

 People have commented already on good books or resources to have in-game mechanics behind kingdom management etc, so I'll take an entirely different approach:

This might not help you at all OP, but you're pretty exactly describing the RTS genre of computer games. Games like Age of Empires / Age of Mythology, Civilization, Crusader Kings, etc all do exactly what you're asking. This is especially in response to your last request, which was a map that you can update as you build things, which you expend resources for. That's the bread and butter of the RTS genre. 

As you've seen from the responses, there are different ways to do this. You need to think about what you want from it.

In Strongholds and Followers, they approach it from the angle that high level parties have a lot of money and nothing to spend it on. A base gives them a money sink and a reason to go out and get more money. Their approach also provides additional incentives to level up their base because it grants them unique powers. This approach is meant to help fill out downtime and just be a fun compliment to adventuring. It's not very deep and doesn't require a lot of time or player investment.

Then you have Midnight's outline above. This approach is much more immersive, but also kind of takes over the whole campaign. You may find yourself spending whole sessions on securing your building supplies or dealing with theft. For a group that came to the table to fight dragons in dungeons, this could feel like the campaign has been bogged down in micromanagement they never asked for.

So you need to think about what you want to bring to the table and what your group will respond to. Do you just want this to be a fun place to go between adventures or do you want it to be a central focus of the game? The answer will determine the best approach for you.