yes but for the exception of a few kits, if you buy rtr, you get a cheaper radio, sometimes cheaper ESC and motor, where as used plus your own stuff you get to pick what goes in and you can easily make it better.
for example the brushless E-Revo, $669 RTR new, but the steering servos need to go, I have owned a revo in the past and the dual steering servos were annoying, always fighting each other no matter how I set them, killed the battery quickly, so theres another $100 for a good steering servo, but you are getting a 2.4ghz radio system which even though it lacks the adjustments of a $300 transmitter, it does the job quite well.
other kits you might just get a regular FM radio or even AM (though I think no one does that anymore on a brushless kit), which you would need to replace.