I've used the same Martiini knife for the past 10 years or so. I don't remember what I spent on it, but it was not very expensive. It's filleted thousands of fish in that time, and will continue to do so.
I like a filet knife to have a good amount of flex, but sturdy enough for the big catfish.
The Mora fillet knives are a good bang for the buck as well. You don't need to spend a large amount, unless you clean fish all day for a living.