To me, $500.00 seems like a perfectly valid offer, and maybe a touch on the high side depending on other circumstances.
As you mentioned, this would be "found money" to the current owner, so that is in your favor.
If the current owner does not use the property at all, I think that is in your favor.
If you had a way to see what the taxes were for the plot of land you want to lease, that could be a good target number if the current owner was even aware of how much he/she was paying in taxes for the idle land.
The other thing that could be in your favor is if you were going to be a good steward of the land. For instance, were you going to build a dock/pier? Are you going to do some improvements? Will you keep other trespassers off? and Will you be leaving the fish in the pond when you are done leasing the land? Does the land need cleaning up?
A lot of the things I mentioned take work, and you can use some of the work as "sweat equity" and hopefully reduce the cash outlay.
Good luck.
