Even though the Mindstorms kit is going to be discontinued in the near future, finding it at a discounted price is tough. We own 6 sets for our Robotics program and had to pay close to full retail for each one, even through
Lego Education.
As far as ebay, make sure to know what you are actually getting, some people have less than the required parts in there kits (2nd hand) and it might actually be better to buy "new" or "sealed" to save money instead of trying to piece together a set.
IMO, I would consider buying the new EV3 set, we have 2 sets and the kids on the teams seem to like it better than the NXT (as it will be difficult to replace the "brain" of the NXT later on as as opposed to the EV3)
FYI: there have been 3 versions
1st RCX
2nd NXT (will be discontinued when they have a way for to sync EV3 for FTC competitions and will not need the wifi (samantha) module, they have predicted by August 2015)
3rd EV3 (released last August 2013)
The good news is they do not often update the the sets (mindstorms have been around I believe for close to 10 years) RCX for about 5 years before that.