It would definitely be in increments of whatever you want. Say it cost $100 per slot (i'm being outrageous here just so I don't promise anything we can't live up to in the future -- I can definitely promise it will cost less than $100 per slot) -- you could buy 1 slot and it would cost you $100, 2 slots and it'd cost you $200, etc. So if you just want one extra icon you will be able to buy it! If you want 20 icons, you can buy them! Etc.

The one caveat: it's not practical for us to run card charges for payments of less than $3, because we wind up losing money on it. So if you're paying with a credit card directly, not from points, there will be a minimum of $3 worth of icons, whatever number that turns out to be. (If you have points in your account and are paying with them, that's fine, you can buy 1 point's worth of icon (assuming it goes down that far, icons will definitely cost more than the $.10 that one point is.))