I have an X and while I don't have a ton of money into it, I bought it brand used off ebay. The engine was a bit rough and some of the screws used were questionable... was pretty clear the guy that had it was new to the hobby and it spent most of it's time on pavement, or he cleaned it really really well. The scratched up cooling head said pavement runner though.
I've added a 47T steel spur, new engine, cheap tuned pipe, T-bone rear wheelie bar/skid, different shock springs, wheels/tires of course, super HD bones/outer axles and sintered BP diffs. waterproof t/b and steering servos... So, a decent sum I guess over it's life.
When I got it, the engine was shot, front diff was hopping teeth when braking and rear diff was on it's last leg. Both diffs were machined 43t ring gear, not the BP's. When I got the BP's off ebay, they had the super HD cups, so I upgraded the bones/outer axles as well.
All that and I don't think I've put more than 2-3 gallons through it. However, I haven't really had to fix anything either. I think I replaced the wheel bearings and cb bearings a couple times.