Not much will come to mind with $50 for me... how fast do you want it to go? You could always get a Monster Stock motor and a esc for local hobby shop or something.
here are a few motors on tower i like:
http://www2.towerhobbies.com/cgi-bin...?&I=LXRR49&P=0
http://www2.towerhobbies.com/cgi-bin...?&I=LXDDA1&P=0
this was one of my most reliable and good to me motors when i was racing
http://www2.towerhobbies.com/cgi-bin...?&I=LXAVJ2&P=0

as for esc i know novak are really good. but not so cheap.
here area few i know will handle those above motors but not as good of an esc.
http://www2.towerhobbies.com/cgi-bin...?&I=LXAXT7&P=7
http://www2.towerhobbies.com/cgi-bin...?&I=LXGZX1&P=7

thats about all i can think of for 50 bucks