How to determine Royalty Values


I own mineral rights on 15 acres in Weld Co., CO. T 5N, R 65 W SE1/4Sw1/4; SE1/4. I have received several calls wanting to buy my the rights but they vary widely. I inherited the rights and live in a state that has no minerals so am completely in the dark about these things. Any help in determining value will be appreciated. Thanks