For sheer filtering capacity and around $100, I don't know of any small filter that beats the Katadyn Combi - advertised for up to 13,000 liters. Ceramic filter, though, which might be a concern. Found it for $84 at moosejaw.com (whom I've never dealt with - just did a quick froogle search to see if it was in your price range). What you buy will be a tradeoff between filtering capacity, what it can do (ie, filter vs. purifier), pump effort, output, and probably a bunch of other factors. I have the Combi, but I've heard good things about MSR, too.
- Dwayne