what is "cheap"
RAIDMAX Helios or AEOLUS - I've done budget builds with each of these.. they're basically the same thing.
both of them i paid $30 after promo and mail in rebate.
they support extra long video cards and have a fan to blow between them, doesnt get any cheaper than this for an SLI build.
There's a plethora of nice mid towers in the $70 to $80 range.
i have a 770 4gb SLI setup in my HAF X.. which is awesome. it has built in fan that mounts between them to blow air in the gap, has the 200mm side fan blowing on the cards, and adjustable support brackets to take the weight of them off your PCI-E slots. but its not 'cheap'