Green Man Gaming has Bioshock Infinite for the PC on sale for $19.99 right now (50% off). If you use the voucher code GMG20-KCDKK-H4TPB you can save another 20% off bringing the price down to $16.
This is a Steamworks title and requires a free Steam account in order to play.
This is for the PC digital download version of the game only. Prices in GMG are in US dollars and exclude tax.