I doubt we'll ever see the ability to make in-game purchases for real money in the PC version. There isn't an equivalent store that will supply the mechanics.
There are many PC games that have the micro-purchases. Generally, you pay through a PayPal account to obtain credits that you use toward the items.
However, I actually prefer to just have an upfront fee and then get everything. Advertising something as free and then having all the little fees is annoying.