Recommendations for a reasonably priced air filter...

Rhino do a few types. The cheaper ones which get sent out when you buy extractor kits for example might only last a couple of months. Just upgrade it to a decent one and you'll not have to worry about it for a year or so

It is a Rhino & I was the cover religiously.

Alright mate, 3 grows is about right for a cheap carbon filter I'd say, the best iv had is 5 grows and that was using a rhino carbon filter. I take it that you are meaning its no longer scrubbing the smell from your exhaust system

Have you changed the cover ?? Usually with the carbon filters you can change the sleeve once and then it will need to be replaced. Any grow shop online will have cheap budget filters mate. £30 for a 4 inch