Amazon is currently selling the Briggs & Stratton 491588S flat air filter (for B&S Quantum engines, among others) for just $5.50 (and free shipping). If your mower uses this air filter, now may be a good time to stock up on a couple. The list price is $13.99, even though you can typically find them for much less than that. As a comparison, SearsPartsDirect.com currently has it for $7.49, and LawnMowerPros.com sells it for $6.95.
The only catch is that these air filters are part of Amazon's Add-on program, which means you'll need to order at least $25 worth of stuff from Amazon (including the price of the air filter) in order to get the air filter shipped, but it will ship for free. So for example, you could order five of these filters, but if you only ordered four you'd need to add something else to reach the $25 threshold.