You are not nuts I've bought extras of vehicles cause I found em dirt cheap for spares... Just I usually end up throwing a whoole setup in them, and using them too, or borrowing em out when I run
Like I bought a 2nd CRT .5 of ofna for $70 cause the body which I wrecked on my original one was $50 at the LHS(before taxes >_<) I was like HMM for $70 I can get a whole spare set of parts wow that is a no brainer. Ended up also buying it a full set of CNC upgrades(not from ofna though as their upgrades should be made of titanium for what they charge), and its own motor
If you think about it they have to package the separate parts, ship em, and the hobby shops have to store em. So if you think about it the fee isn't that insane, but yes I will agree its a tad high. But alas they have to also stock the not so frequently bought parts too so I'm sure some of the cost is thrown on to even those out.
But seeing the main things that break on RC's are A-arms, and knuckles (if they are plastic) in a way buying a 2nd RC doesn't even the cost out cause if you break the arms you paid $200 for a set instead of $6 lol
Now what is REALLY ludicrous are those (how one guy who used to work at my LHS put it) *magical* durobro (or whatever their name is) screws hobby shops sell that are 4 for $2
Also if you want to see insane markup look at HPI Recon parts bags I broke 3 parts on my recon to get the 3 parts I needed I would have had to buy $130 in parts bags for a RC I bought for $90 NIB...