I made one out of a cheap 8" Radio Shack sub mounted to an old snare stand I had layin around. Works fine but don't forget the ibnline pad. (aka attenuator) The subs output itself is too hot without it.
The one I bought was like $30 and it's from Shure. Adjustable db drop from -15 to -25.
I track every song with it but not every song will have it in the mix. Some songs it fits, some songs it don't. But at least it's there for another option.
my 3 pesos...