coupon code: 1GFT82012
Gamefly has Quantum Conundrum on sale for $3.75 (75% off). Use coupon code 1GFT82012 for another 20% off, bringing the final price down to $3.00. Coupon codes are entered on the final order review page.
Once Gamefly has invoiced your order, access your Purchase History under My Account. You'll find the Steam activation code there.
This title has never been priced below $8.99 before (which was during the Steam Summer Sale), so this is a steal.
Please note that Quantum Conundrum requires Windows 7.