Well, they have some nerve proposing to charge you when they're hoping to run pipe through your land. Hardly an incentive !
If you can contact whoever's hauling the water and find out what they're charging, that would help your figuring.
Seems to me your fee needs to cover land value, loss of use of the land their pipe & whatever will occupy, and whatever interference results from maintenance.
Getting a piece of the action on top of that wouldn't be unfair, and keeps you in the info & consideration loops.
I'd start with either free water, or enough for you to do your own 2nd well (probably the simpler & better option).
You might want to do some research on "eminent domain", "land rights", etc on Wikipedia.
Also, if you enter into a contract or agreement, if there's an attorney near you who's good with land use, rights, etc, I'd consult with him/her as a CYA.