Rule of thumb would be 220 cfm for each antminer and maybe about that much more for the GPU rug. That might get you into the neighborhood of 3000 CFM for your setup.
If you have a LOT of natural ventilation / open windows etc. you may be able to get away without a lot if forced make-up air, but to be safe you'd want enough air being forced into the basement to replace the hot air your forcing out.
A cheap answer to getting ~ 3000cfm might be something like this:
http://www.globalindustrial.com/p/hvac/fans/blower/portable-ventilation-fan-12-inch-with-16-feet-flexible-ductingYou can order a 12" exhaust that looks similar to an oversize dryer vent through Home Depot to cap off the outside, or you can just shove the exhaust hose through a window if you have access.
