Green Man Gaming has Bioshock Infinite on as a 24 hour deal for $29.99. This is the same price as on Steam. However, you can add the 20% off coupon GMG20-F202F-UI40F to bring the price down to $24, saving an additional $5.99 off.
Bioshock Infinite is a Steamworks game so you will have it on Steam regardless where you buy it.
This is for the PC digital download version of the game only. Prices in GMG are in US dollars and exclude tax.