I agree with comments about going 2nd hand, but if I read your post you are actually asking whether you can build a low cost car for similar money to a rtr and be competitive?
I can confirm that it is possible to spend lots and be uncompetitive
so presume opposite is possible.
Joking aside, when I race at cullingworth there is (or certainly "was") a chap who raced a hobbyking bz222 (or whatever it is called - their 2wd that looks like a Durango). The car is cheaper to buy than my rear hubs on my yz2. Every week he thrashed me.
A cheap car with cheap electrics in the hands of a decent driver will beat most people most of the time, but then you have the dilemma of wondering whether you could go faster with a better car (which is what manufacturers have relied on since time began...)
I'd still suggest a decent car 2nd hand, but if you want to go new and low cost then hobbyking is your friend. Even if postage costs aren't.
Just as a final point, when I can back 3 years ago, I bought all team c for my son and I - 1 rtr and 1 2nd hand. Both were great but not very resilient - I would budget for 2 front wishbones per race meeting
We now run yokes - yz2 and bmax2 and simply NEVER break anything. Parts resilience is a world apart.
A bad car set up well and with good tyres will almost always beat a great car with bad set up and "wrong" tyres