I just finishing putting together my 8ight-T roller. I'll give you a breakdown of roller cost vs. the RTR. I'm listing the components that would come with the RTR version.
8ight-T roller (1.0): $350
Go 7 port .21: $210
Pipe: $65
Spektrum DX3R radio (comes with receiver): $300
2 x Futaba S9351 servos: $240 (combined)
NMIH battery receiver pack: $20 or so
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$1,185.
The first thing to note is that I put relatively high-end electronics on mine. You can save money by getting lower end electronics. You can get adequate JR servos for $100 (combined). My engine is a mid-priced one. There are a few engine choices around $150, which would save you $60. You can get a lower-priced radio, too.
Another thing to note is that I didn't spend $1,185 at once. I used the electronics that I bought a while back and were in my retired Mugen. I would have used the same engine, but it was worn out and I decided to buy a new one. In theory, the real price that I spent on the 8ight-T roller was just the roller itself: $350. On top of that, I will be able to sell my MBX5 roller. An 8ight RTR stripped down to a roller isn't worth much at all.
My point is that the initial cost of an RTR is inexpensive compared to buying all of the components for the higher-end rollers/kits. But further down the line, RTRs end up costing more. The radio and perhaps the engine in an 8ight RTR are worth transferring to a new car, but little else is. I'll stick my components in my next kit, too because they're going to last, and will again be cheaper than an RTR.
But, I race my 8ight buggy RTR too. It does well and it's well worth the money. I'm just giving you the other scenario as well.