1996 GMC Jimmy Freeze Plug Repair

I have a 1996 GMC Jimmy that was bought used over a year ago. It is leaking coolant from about center area on the passenger side of the engine. I can not see to be sure but assume that it may be a bad freeze plug. What is the approximate amount of costs for this type of repair? Or could it be another leaking problem? I do not think it is a gasket as I have used stop leak and that has not stopped the problem. The coolant seems to leak out quite a bit when the engine runs but stops after it is turned off. Thank you for any info you can give me as to possible estimate of repairs.