There are many inexpensive knives suitable for panfish. A blade in the 4" range works well for that size fish and I would avoid anything in the 6"+ range. I like the vintage Rapala (Made by J Marttiini) that can be found for under $15 regularly on the secondary market as well as the Mora fillet knife. If you would like a folder, the Opinel Slim series (#10 is about the right size) are excellent and low cost. You can spend a lot of money on a fillet knife, and I have many times over, but you don't really need to.