Acemart,
I know a cheap way.
For about $35 you can buy a wireless fan remote that turns off/on depending on the temp (I found this remote at home depot). The remote turns on low and then high has the temperature rising and visa versa. The remote needs to be placed in 2 or 3 zip lock bags and theb you place the remote into your sump. You are going to want to replace the plastic bags every couple of months to play it on the safe side. If the remote gets wet you are then going to need to get a new one.
I have 2 fans that are on the remote and 2 additional fans that turn on and off with the lights. I did it this way so just in case the remote does not function, I still have some heat protection from my 2 - 400w mh's.
You are also going to need an extention cord. You have to cut the cord and then insert the wireless fan operating system (fos). The side of the cord that plugs to the wall outlet is connected to the power source of the fos
and the other side of the cord to the fan side of the fos.
You are now good to go. Now all you have to do is set the settings on the remote.
ps. do not forget to put batteries in the remote.