The general answer when this question is asked is to check eBay. That is the best way to get a real assessment of what items are actually selling for by searching through the completed listings.

That said, you will need to know exactly what you have before you can accurately assess their value. There are a lot of packaging variants for the original 4 Turtles. And the original molds were reissued several times over the years since then.

If you need assistance, let me know. I can probably give you better idea of what you are dealing with if you send me a photo of the front and back of the card.