I've never offered anything up front outside of letting people know I will return anything that is significant to the property/house itself or their family. The 1850s house I detect at, I've given the owners some of the relics I've found just as a thanks for letting me hit their property whenever I want. Honestly, MOST people who are okay with allowing you to detect will be interested in the things you find but won't ask for anything. If I ever have anyone following me around and expecting a cut I won't be staying at that spot for long... It takes a lot of work to get to the good stuff, and I'm not about to just hand it all over to the land owner who hasn't put in an iota of work for it, ya know?
One thing that I feel is important when asking people for permission to detect on their property is to emphasize the historical aspect of it and your interest in any relics you may find, that you will carry out any trash you dig up, and that you'll leave their yard looking just as good as when you started. Try to put their mind out of thinking that you're just trying to go strike it rich in their yard or something, because if someone gets it in their head that you're going to be digging up silver and gold coins on their property left and right (not likely) they're going to be a lot less likely to say yes.