Cheapest Xbox currency

I have $14.55 in my Xbox account. Apparently, if I want to purchase a game that costs $15, I can't take the money out of my account and then charge my credit card for the remainder. So, I have been looking to see what the cheapest Xbox currency is and where it is available.

I found Bing Rewards has a $3 digital gift card, which I will be able to get with reward points after like a month. Is there anything cheaper? Are there $1 digital gift cards? Searching for this stuff just leaves me with lots of spam links.

Xbox Rewards will put $1 into my account, once I rack up 1000 points. By just doing the surveys, it will take me 4 months to rack up that cash. If anyone is looking to go Gold on their Xbox, a referal link from me will net them 5000 points ($5) and 1000 points to me ($1). That would be an option, but my friends already have Gold.

This is mostly out of curiousity: where is the cheapest place to buy Xbox currency and what is the cheapest price? Thanks.

To my knowledge five is the cheapest you can buy since Zune went under. You can buy stuff on the marketplace on the console with it only charging the reminder (.45 in this case) to the card you have attached. 

Edit: you can probably still find someone trading a GS Promo 5$ code for about 3 bucks.