Before you buy this READ THIS
Please read your manual. I used to work in a gear shop, and every last one of these we sold was to someone who didn't need it, or needed it because they mistreated their old filter.
Step 1: Don't need a new filter: I've had a MiniWorks for 11 years, if you let it dry out after trips (prevent mold), don't let it freeze with water in it, and make an attempt to get clear water into it, it'll last a decade or more.
Once you think you need to get a new one of these, check the MSR manual (online too!), and scrub off the surface according to their directions. Presto, I probably just saved you $40. Scrub it off, and when you do that, just bite the bullet and get new O-rings for it (normally less than $5 at your local MSR rep).
Also, make sure that you've got the prefilter on the little floatie. That'll again extend the life considerably.