I understand your frustration OP and the “venting”.
That said, you do need to move away from the land only being worth 10k.
As pp’s have said, it’s actually liable to be worth more than that because of the value it would add to your property - so that needs to be your starting point.
Get a valuation so you know what is a realistic fair price and then decide if you can afford/are willing to pay that.
Then if you want to proceed go back to them with an evidence based counter offer and make clear that’s your ceiling price.
I’d also do some checking and make sure their claims about inheritance tax are valid.
Ultimately if they don’t want to sell for then you’ll have to accept it.
I get it’s frustrating. My neighbours have a small cottage on their land (that’s nearer to our house than theirs that they never use) that we wanted to buy.
They wanted stupid money for it and so we declined. 5 years later they have decided to move and due to the market are struggling to sell. They’ve now approached us to buy (at a more sensible price) as it will help sell their main home as they can reduce it accordingly.
Problem is, the main reason we wanted it (as overflow when older relatives stay) has gone (sadly as a result of bereavement).
So this time the shoe is on the other foot and they are now apparently kicking themselves that they missed the opportunity to sell (according to mutual acquaintances) previously.
So you may well find circumstances change in the future and even if they don’t accept a sensible offer now you can just leave it on the table and tell them to come back to you if they change their minds.