Prius Plus

I've got a 2001 Prius with almost 90K miles and I'm thinking this would be as good a time as any to try the "plug-in" modification to the car. Warranty is about to expire and the car has served me well.

However, so far, I'm only finding a $10-20K price tag for the Prius-Plus conversion and I've not seen anyone in San Diego give it a shot. The price tag is too high (can't justify more than $3-4K for gas savings) and I'm not sure how the car performs after its tinkered with. Any clues would be appreciated.