Take the price of the ARF you want (different factors for a kit built)
Add in the price of the engine
Add in the price of your radio

Throw in from between $75 - $150 for *extras* like props/fuel/fuel tubing/spinner/starter/glow plug igniter/field box/power panel and stuff like that.

If you've never really thought about R/C and are just *interested* go out to the local club (or clubs if more than one) and talk to some people and watch. Flying is FUN but requires some work and some people shy away from that.

Plus some clubs (mine does) usually has some guy with a beat up trainer that he lets just anyone fly once or twice to see if it's even fun for you.