Amazon.com Has BioShock Infinite for $19.99 – this is for the downloadable Steam code, and MAKE SURE you only select the downloadable Steam code option.
The Steam code will allow you to select either the PC or Mac download.
It was released about a week ago by Aspyr, and this is about as cheap as you will find it. I have no idea on how long the sale will last, but it’s half-off.
There are some hefty requirements, starting with 30GB of HD space and OS X 10.8.4 Mountain Lion.