Here's a short DIY article with instructions on how to change the cabin filter in your Prius. You can literally change the cabin filter in less than 5 minutes. As a matter of fact, I changed my cabin filter while on my 10 minute break at work and it took me a little over 3 minutes.
87139-30040 - Element, Air Refiner
87139-50100 - Premium carbon activated cabin filter
Empty and lower your glove box.
Apply pressure against the hinge located on the right side of the glove box (closest to the passenger door). This will loosen the hinge from its socket on the dashboard. Carefully maneuver the glove box out of the dashboard.
Pop open the lid of the cabin filter housing by pressing in the push clips on both sides of the lid.
Remove the old cabin filter from the cabin filter housing simply by pulling it out, and then replace it with a brand new filter.
Return the lid to the cabin filter housing and press down until the lid clips onto the housing.
Re-install the glove box by reattaching the hinges to the corresponding sockets on the dashboard.
Pretty simple isn't it? A genuine Toyota cabin air filter will probably run you around $14-$40 on discount Toyota part sites, depending on your choice between regular or premium carbon activated filter.
An aftermarket cabin air filter will cost you a mere $6-$20, again, depending on your choice between regular or carbon activated filter. You'll be saving some cash with either route you go with as an air filter service at the local Toyota dealer can cost you anywhere between $56 - $65.