One free site is eBay's "Completed Transactions" page. First get on the page with a particular coin, then, in the left margin of choices click on Completed Transactions. You will most likely find several recent sales' prices (current market value) for your type of coin--then adjust for condition.
Don.....
I haven't had to pay for initial searches at PCGS.com. One of the better coin value sources, based on some excellent grading. Values reflect slabbed/graded coins only, though.