It won't show it as an error, rather just the form to fill in again. I guess you could call it a bug, but it's not that hard to add the functionality. Just another if(){} statement.
As for the actual cost, I don't know the current valuation of points to dollars. Microsoft does like 850 points for $14, and then when you get to 1000 it's like 12.95 or whatever. So there's no steady 500 points for $5. That's something you'd have to link to.
What you could do (I guess) is to constantly update the prices manually, or get an RSS feed. Then just get the Least Amount Needed to have a larger supply of points (i.e. if it would cost 875 points, you'd be best to get 1000, or 850 and 100).
Since you don't know what each person is going to want, and I'm sure that it's a sliding scale of prices, it'd be tough to do.