Resistance: Retribution is $9.98 New at Amazon.com. I've been waiting for a price drop on this for a while so I thought that I would share.
http://www.amazon.com/Resistance-Retribution-Sony-PSP/dp/B001COTC3E/ref=sr_1_1?s=videogames&ie=UTF8&qid=1297290975&sr=1-1#productPromotions