Hi folks, I've opted to give my party a very rural bastion in the form of an abandoned hill farm!  The original farmhouse is small and basic, as befits a starting bastion, with a nice derelict wing (with basement) for expansion into and to add all the bastion extras one needs.  There's also derelict housing for soon-to be staff and workforce. The party have privacy, wood to build and cook with, access to a tarn to fish in and the ability to crow crops, husband animals, build stabling and generally become self sufficient over time and earn a few coins.

What I would appreciate is some ideas on pricing.  Things like costs of building work to repair the derelict bits and staff cottages, livestock costs & feed, seed for crops etc. Also, what sort of yields would be reasonable for a small hill farm?  Any ideas or thoughts appreciated!

Many thanks.

Are you using the ‘24 bastion rules? It’s got pricing for expanding the various facilities. 
I’d stay away from adjusting them based on the location. D&D doesn’t, and isn’t meant to, simulate real economic factors in pricing. 
If you’re not using those rules, you can really basically pick numbers. Personally, I like to say the place is self-sustaining. It makes as much as it needs to support itself. Otherwise you start running some kind of small business simulator minigame.

Also, most remotely plausible business investments take years to decades to pay for themselves, so unless you're planning to have an extremely slow paced campaign a business will not make a profit over the course of a campaign.

The vast majority of peasants in D&D are owners of small farms. Returns on farms has always been tiny, and for most of the year your in the red waiting for harvest time. Plus on bad years most farms lose significant amounts of money (hence why farmers were among the first to develop insurance). A wealthy noble would gain their income from the rents on 50+ properties/businesses. So if we use D&D income levels, and say a noble makes 15 gp per day, that would make profits per farm to be ~3sp per day over an entire year. 

While all the advice here is accurate there are exceptions I had a gtoup win a naval batal against a pirate ship the ship that was stacked 60/40 against the group (they played well) made enough money to buy a large bar in a port town -  i think we ended up on 200gp a week in profit = nothing crazy but 200gp a week in income,  Significant but not broken. 

Play every situation in the way that makes sense for your campaign and level - use bastion rules as a guidline - then use yout brain lol 

One of the main points of a stronghold is that it is a money sink, not a source of wealth. It's a way to turn all the party loot into other kinds of benefits like information, making allies, and just the fun of building something that you can call your own.

As such, I would make the rewards qualitative rather than quantitative. If they have some cows, let them carry around some milk and see what they do with it. Or for something less open-ended, let them use the farm to prepare a home-cooked meal that they can eat during a short rest to gain heroic inspiration.

Rather than focusing on the nickel-and-dime values of purchasing feed & seed and selling goods, focus on the relationships built through these transactions. Maybe their fresh produce impresses a merchant who can provide some information or other quest-related benefit. Maybe the farmer providing feed has a plot hook leading to a stash of treasure guarded by goblins in a nearby cave. Maybe they can use their relationship with the cheese delivery guy to smuggle themselves into the baron's castle.