For me it's $500, for my neighbour it's $5000 - because it stops rabbits from eating his garden and stray dogs or whatever.

If it's $5000 value to my neighbour, should I have charged more even though I would've accepted $500? Who would have been exploiting who then?

This is directly analogous to my employer as well, except replace "neighbour" with boss, and "fence" with code. It's still a voluntary trade.