I just finnished a project like this yesterday and i think it cost me a total of $60.00 US, I used 4 fans, two 3" fans blowing into the top, and two 4" fans blowing out of the top. I am hoping that the bigger fans will create low pressure inside the hood and in effect move more air.
The fans were about $8.00 each and i bought a 12v power supply from radio shack for $20.00 or so. Just make sure that the power supply has enough amps to push your fans. i will post a pict later today.
most comp fans have 3 wires, red, black, and yellow. The yellow is the sensor wire and you probably do not need it.
this was a very simple project.
this was for my 125 gal tank