Most of the time if you are going to go through all that work to upgrade a cheap car it would of been better off just buying the better car upfront. I personally would just save the money and get what you really want instead of just buying a cheap car and throwing money at it. For example you can get a HPI Hellfire and throw tons of money at it and make it better, but it will never be like the top vehicles today and still be classified as a hellfire.
If you are talking about say a D8T and a MBX6T. Yes the D8T cheaper, but is a car capable of winning. Same with say a V-Spec and a Speed. The V-spec isn't as good as the speed in most cases, but the V-Spec is an awesome engine. You don't need to buy the best of the best to win. Driving will come into play more. For example. I bought a 8ight-T 2.0 RTR. It isn't a nice as the roller. But is a solid platform capable of doing well. In this case putting some money into it this vehicle as it breaks or upgrading a part every now and then is worth it. It all depends on the platform and vehicle you are talking about.
What car are you talking about?