From $30 to $300. That covers everything from Bburago/Maisto to semi-mass produced higher end 1:18's, like Exoto, AutoArt, GMP, CMC, and, more recently, BBR. AutoArt offers the best car for the money; CMC and BBR are in a tie for top dog. IMO, of course.
There's no upper limit, but anything below $30 is most likely toy store crap. Lately, I've been slowing down and saving my pennies for a better class of model. It's the old quantity vs quality question; some people would rather have one great car than a hundred ordinary ones.
Didymus