Hi Sarah,
I still sell cards and they vary in price. I do most to order now where people like the work you've done and order something more specific to their needs. These bespoke card requests usually come with their own price range as the person will tell you what they want to spend (usually more than they would on a general card).
I started with geenral parties for family and friends - they're the most honest and won't order if they don't think it's worth it. Have a look in the shops for cards similar to yours and knock of 30%ish. Like Diane says - don't underprice to start with. That said it's important not to go too high. The one thing with earning from your hobby is that peopel never expect to pay an hourly rate and if that's what you expect you stop enjoying the making.
Find your market, Match your items - as best you can - to the current market with a reduction and see how they go. It can take a while but friends tell their friends who tell their friends and it's nice when the requests come to you.
Good luck
Kim