I Started with the Team assoc. RC10GT RTR so I think I can help you understad a bit better about the how and why as far as $$ go.
With the RTR you get;
#1 the motor... Hard to keep tuned overheats won't last long you will either quit or buy a new motor soon.$100-$300
#2 the Radio I was actually happy with it but the recievers are VERY suseptable to water and the inevitable wall. after you break the reciever and the servos several times you will buy more expensive servos( go with metal geartrains) and a better transmitter/reciever FM.$120-$350
With the kit the parts that are included are of better quality
(Break less easy everything breaks eventually) Your inital startup costs are going to be in the $500-$1,000 range but after upgrades and breakage with the RTR you will easily spend that much to get the car to run reliably. NTC3 is a good vehicle and parts are everywhere. I have seen alot of very nice kits all built and raced for sale in the for sale forum on this site in the $300 to $500 range most with hop ups and or motors most without radios.
I have been planning my first kit for about a month now and this is my plan starting this weekend;
#1 kit MTX3 or R40 flip a coin at my LHS.MTX3 is better R40 looks easier to set up.$260+hop ups-$360
#2 Radio airtronics MX3 no servos$90 MG 525 or eqiv. servos $70-$80($160-$170)
#3 body shell+ paint $40
#4 Motor RB 12v 3 port#410 $220
#5 starter box $75 tempgun $30
#6 Time and Luck for building the whole thing .......Priceless.
So as you can see it all adds up and if you do it wrong to start it will add up to more in the long run no matter what you do its going to get expensive.Nitro R/C cars are a Big black hole in the Pavement that you dump $20 bills in everytime you look at the thing. But its Fun............ total bill before Fuel $800-$900
Good luck!