Finding a good cheap speed controller for brushed motors can be hard, especially for small robots. I have been on the search for a good supply of low cost small speed controllers for the past few months and I think I have found a solution for under 7 dollars.
There are a large number of small brushed controllers made for TZ4 type RC cars. These controllers are sold under a bunch of names, but tend to be all the same. The problem is these controllers run in a mode setup for RC cars known as Forward/Brake/Reverse. To switch from forward to reverse you need to apply reverse twice. While this might be fine for an RC car, this doesn't work well for tank style (skid) steering robots. Some models of these controllers can be purchased at the links below.
It turns out there is a way to change the mode on the speed controller to a more robot friendly Forward/Reverse. It requires simply soldering a specific jumper on the board. The image below depicts the jumper that should be closed. It is the jumper located between the power switch and servo input. There is a small arrow on the board pointing to the jumper.
I have done some testing with these ESCs and they appear to perform well above their rated spec. I have tested them from 5.0v to 16.5v and found they perform without issue. They also have a 1A linear BEC for powering your receiver. All in all pretty great for under 7 dollars.