In my current game my half-elf sorcerer just gained a title for some land due to her actions. The land itself is pretty much undeveloped farmland ATM but I plan to change that. Firstly by having our druid enhance the lands fertility, then scouting for minerals and metals in the area, developing and exploiting them to form viable industry, and finally pouring the income from that into research to develop both public and magical schools. The thing is that the land is also near a lengthy desert and, on the opposite side, there's both a dwarven and elven kingdom effectively cut off from the larger world thanks to the desert.

I want to change that by establishing a trade network with them and, would you look at that, my sorc just hit ninth level. I figure I'll first develop my land so we can afford to construct the circle (18,000 gold is nothing to sneeze at, 36,000 for a working pair), but once it's done merchants can utilize the portals (for a small fee) to instantly travel between the two other kingdoms and my new county, skipping the desert (and both it's lengthy delays and lethal climate) entirely. I figure even if I front the cost entirely they'll spend enough in my county before heading out to other trade routes that the economic boom will more than offset the portal cost. The thing is... I'm not sure how feasible this actually IS rule-wise. I'm not worried about the cost since, ideally, the increase in trade will make up for it, but that only works if it's actually doable. Is it? What would this network entail?

I will first point out that teleportation circle doesn't just connect 2 circles but could be used to travel directly to and from these cities if they have a circle themselves (and you/merchants know the sigils).

Second, yeah this is totally fine by the rules. You might have to RP with the DM to establish the new trade routes, but the circles? No problem.

*also, 36500GP. Spells don't take off for holidays.

Sounds like a fun base building project. I wouldn’t worry about the rules, if your DM is game with this working in their setting, I’m sure they’ll make it worth your investment.

Teleportation Circle is a 5th level spell, however, and even creating a permanent circle does not allow one to use it without casting the spell, just allows you to anchor it as a destination that other circles can connect to. Depending how permanent circles work in your game, your DM might let them be activated by laymen with passwords or keys, but... RAW, you cast the spell every time you use one. 

Regular merchants probably don’t regularly travel with a 9th level wizard... but if it’s an important enough shortcut? I’m sure that larger trading companies or kingdoms would find a way to work it into their supply chains. Whether that’s a boon to you, or a reason you may need to defend your home from those that want to steal such a valuable  waypoint... who can say. 

Alright. That means we'll need a way to cover the costs on a consistent basis but, assuming that's viable, I'm more concerned about the number of people/how much goods can be teleported. If it makes a portal than, for even just a minute, the amount of goods that can be shuttled through in a rapid transit would be worthwhile (if extremely limited). If only one person can use the portal per cast then we have an issue as the amount of co-ordination to make such a thing viable may not be enough to justify the cost from a trade perspective.

It’s a 10 foot diameter circle, so probably one or two wagons at a time could pass through... but it’s only open for about 2 rounds, so a couple wagons, or a small group on horses or foot, is probably all that can pass through at a time before you need a second cast. 

The spells as presented aren't really set up to do what you're asking without multiple high-level spellcasters. Even then it'll cost a lot.

If I was in a remote location and 17th+ level wizard, I'd use Sending to set up the basic trades , then when everything is ready I'd Teleport there, set everything into wagons: open Demiplane, wheel the wagons in, go in myself. Then cast Gate to continue wheeling everything right back to my home.

Or if a Chronurgist wizard without too much in need, - use Sending for setting the trade with a trusted merchant: and use Galder's Speedy Courier to put money in as well as using Arcane Abeyance to put in a spell mote of the same spell: the merchant takes the money and mote, then uses mote to cast the spell so they can send the ordered goods. It'll cost 50 gold plus goods/service, but less sloppy than casting Teleport Circle twice for twice the gold.

Or just work with the DM to invent something. The spells in the book are not supposed to represent all available magic. They're just there as convenient examples of basic magic you can select in levelling. You should be working with the DM for something more complicated/practical for your needs.

Permanent Teleportation circles are presented in-game as being useful for what Snowtworf is describing, its just that the actual mechanical function of the player spell Teleportation Circle doesn't really permit anyone but adventurers to use them. There's precedent in published adventures for NPCs allowing the group to use their circle for 50 gp/party member (probably about 200-300 gp for a cast), which is a bit lower than the expected 1100 gp cost that the formula underlying spellcasting services in other publications suggests would be appropriate for a 5th level spell with a 50 gp consumed material component cost. 

One way or the other, Teleportation Circle is intended to be used to create waypoints for fast travel, but is not really set up for low-level or non-adventuring NPCs to take advantage of for routine trade. Other plot-magic that uses a circle that's been made permanent by  the group usingTeleportation Circle might be appropriate, since those circles do interact with other spells like Teleport and Plane Shift.