A VEX kit would be giving you the most bang for your buck here and it's the closest thing to building an actual FRC robot that you can/could buy locally. Seems that Radioshack's online store has permanantly slashed prices in half and they still have the kits in stock
Right Here.
However, I do have to say, if it comes down to getting an RCX or an NXT, go for the gold and get the NXT. In my opinion, it's going leaps and bounds beyond the old mindstorms kit and any team who utilizies its functions will have a pretty decent advantage over those with the old Mindstorms kit.