my buggy is a mp777 sp2 with a novak HV4.5 motor, 2 aluminum battery trays and (the biggest weight issue) 2 2s 10,000mAh lipo batteries.
I decided to go with the huge batteries to ensure I could survive the 15-20min A-mains at the local nitro track, as well as bash for ever. Although going with say a 5000mAh battery would have cut .8lbs off of the weight of the buggy, I didn't know for sure if it would give me enough runtime. (On occasion, if someone needs to leave by a certain time I have seen them run the race right after that classes 2nd qualifying, which would add 5 minutes onto run time.
Cost effectively, if I refuse to cut down on the batteries the wheels (if I can find something a little lighter then those 0.86lb wheels that aren't to expensive) are one of the few things left for me to cut down on, other then maybe remaking my battery trays from UHMWPE or derlin. Or changing motor/esc as the novak system is heavy.
I do need to run the buggy more, once I rebuild the diff to see if my motor thermaling was a fluke (I was running in grass) or if it will actually keep happening as thermaling is just as bad as dead batteries in a race...

I could try to put another fan or two on the motor... or maybe buy novak's new fan mount thing as I am pretty sure it would save weight
Here is a pic of my buggy, other pics are on my "mp777 sp2 brushless" thread