Leasing mineral rights in Southeast Laramie County

I was just contacted by Westphal Energy working in conjunction with Chesapeake Energy yesterday and they made an offer to me yesterday, because according to them I own 20 acres of mineral rights on my property. My question is this: What is the current per acre market on a mineral lease as I have heard different amounts but the amount that they shot me yesterday seems a little low as I had a friend of mine who leased his mineral rights for that amount back several years ago? Also, what is a normal percentage for a production percentage?