With some creative soldering can't I make the equivalent of an H bridge using simple relays to route the current and have a cheap bidirectional DC motor controller?
Certainly you can if your directional reversing rate requirement is not too demanding, one can make an equivelent to an H-drive using A DC voltage source, a single DPDT relay, and a single MOSFET rated well above the voltage and current requirements of the motor to handle the PWM speed control. The relay will control the direction of rotation and the mosfet will control the variable speed from stop to full speed. Of course a relay driver transistor would be required to control the higher current relay from an arduino output pin, but that's simple and cheap. A logic level N-channel mosfet would be the best choice when using an arduino also.
Also, why are R/C ESCs so cheap compared to, for example, the Pololu motor controllers?
Mostly a matter of scale of volume I would think. If Pololu sold as many motor driver modules as the Asians sell to the R/C planes, helicopters, and car market I would suspect they would be more price competitive.