Welcome to the forum!
I hate to say it, but if you are new to working interest, walk away, quickly, on this kind of deal until you have spent quite a bit of time learning and thoroughly understanding both the profit potential and more importantly, the risks.
Working interests are not for the uninformed. You need to have deep pockets as the bills can keep coming for years and years and affect your family. You need to have an experienced oil and gas attorney and accountant. You need to understand the kind of insurance that you will need to protect you if there is a serious accident and you get sued.
Factor their fees into the cost. This is not a one time payment. You will get Joint Interest Billing for all operating costs and abandoning costs on this well and possibly others. The current operator (do you know their reputation?) may sell to another operator that might not be very good and may waste your investment and bill you unexpected amounts. Do you know the reservoir? Do you know the likelihood of success? (confirmed by an outside party-not the one trying to sell the deal) Do you understand the oil and gas business thoroughly? Do you understand the tax liabilities? etc, etc, etc. Be very cautious!