I think this is an extraordinarily bad way to get the bonus miles unless you are against the deadline for meeting the spend. The best you can do on a site like plastic jungle is 91% (they claim 92 but I have never found a card that goes that high). Target or Walmart will fetch 91%.

So for your $2500, you would lose $225 on the transaction. Granted, if you want to cost it out, that's 0.4 cents per mile, but why would you give up that money? Look at paying your cell phone bill, groceries, or other utilities with your card to get your spend up. Or at least do some early Christmas shopping.

I understand if you are trying to manage cash for the lowest impact and don't want to spend $2500. But look for items that you need to buy anyway. I would only go the gift card route if I am in month 4 and and short on meeting the goal.