Well, the small items like that aren't something I would base my living on, but they are something to get the name out there and put some bread in the cupboard.
That being said, if the craftsmanship is there, I'd say a better price for something like that would be $10 w/lighter included. I know you were just using the lighter case as an example, but you also have to remember to take your non-project specific time into consideration too. What about the time it takes you to sort through suppliers and find the best deal on products to use? How about driving/gas time to the post office to ship the items out or drive to a booth? What about the time you spent asking this question on how to better run your custom lighter case business? It seems like nickel and diming things but they are all valid "costs" associated with getting those little lighter cases out the door (and any other item you happen to create). And really, a cool Bic lighter case for $10 is a good deal.
There are lots of other discussions on pricing and the various formulas people use, but in the end you gotta charge a price that suits the item while still giving credit to yourself as an artist and the overall amount of time you put in. I started using an Android app on my phone that tracks time and where it all goes (as well as p/project, expenses, tax, revenue, etc...). It's called FTS Timesheet and really opens your eyes to all the stuff mentioned once you get it setup right.