not quite the same thing, but we've been shopping for small holdings this year and I can say with hand on heart that a 3 bed house with 3 acres is often the same price (or even more expensive) than a 3 bed house with 30 acres!
Land / smallholding prices are totally nuts at the bottom end of the scale. If selling a block of 100 acres of agri land, then it's much easier to value, but "smallholding" type land is a law unto itself.
The best you can do is look through uklandandfarms website at the "sold" properties and see if there's anything similar in your county.... same with zoopla, rightmove, etc.
"It's only worth what someone is prepared to pay" is the line often trotted out, but with city & foreign investers in the mix, I don't think it's a very useful expression any more.