cheapest one off ebay and and one that was about standard cost.
they both look exactly like this one: http://www.ebay.co.uk/itm/3D-Printer....c100506.m3226

We figure the BB's heatpad will draw around 27amps :-)
Well I say 'we' - My mate dave, who is an electrician, worked it out and went: 'bloody hell that's going to pull 27 amps !'
At which point I started to think running it through the motherboard probably wasn't that great an idea and recalled autowhiz's posts about ssr's (really mosfets) :-)