You should probably check out /r/MechanicalKeyboards' buying guide here.

Like /u/LuxorSlopski said, the Monoprice keyboard is probably the cheapest one you can get readily, but they use Cherry MX Blue switches which make a click sound when pressed. I think people normally recommend Black or Red switches for gaming since there's no bump when you press them, but Blue switches should be fine if you don't mind the click sound.

Also, if you don't mind a membrane keyboard instead of mechanical, I've heard pretty good things about the Logitech G105. It's about $40 and has more features compared to some of the cheaper mechanical boards.