Looks like a temporary markdown(reg price is $19.99) Just noticed this last night and snagged it at $9.99 to replace my UMD copy.
I have owned a PSP since release and this remains as one of the best/most played games the system has to offer.
GTA: Chinatown Wars was just marked down to $19.99 on PSN.
I missed the last sale when they had it for 15.99, and they bumped it back to 29.99, so Ive been waiting for it to drop again. FUN game and it only takes up 527 MB of your memory.