Sounds wide open subjective to me. I'd dig a little deeper on how they arrive at "value added to the land." Nobody knows what their place is worth until someone hands you a check. If there are any neighbors around the added a similar building, that might be a conversation to have as well. See how theirs changed.
Also, how much building can you get for $20,000? I could see if you get a sweet deal on labor and can do a bunch of it yourself. I had a pole shed spec'd out a few years back, and I couldn't get a base structure (dirt floor, poles, walls and roof) up for under $42,000 for a 32x40. Morton came in around $90,000 for a similar building with dirt work and heated concrete.
If they benchmark off the value of a morton structure, your valuation could go bonkers. I'd get a plan and take it to that dude for a face to face. $500 ain't so bad, but if that mushrooms to $2,000 or more (and then 10% inflation from there), the soul's ability to swallow that, should your interest start to wane, is gonna be challenged.