ok i cant make a tut but ill try to tell you what i did
i used UMDGen to open my cso of FR
i extracted the hole game
open PSP_GAME then USRDIR then Towers
then find any .tower file open with note pad and change the cost to 0 or whatever you want
just dont make the go guns do damange it seemed to lock up my game but feel free to try new things
then just repack your .cso and load it on your stick and play your hacked game
as soon as i fig out how to edit the map files ill post it till then enjoy
feel free to ask for help ill do my best to help so have fun and death to i pod long live PSP
ps take a look at the vid i posted to see better how to change the .towers files better the vid is how its done on the i pod but is the same sort of on the psp