I believe I can answer this.

The problem lies in the fact that Droid/Apple takes $1 out of the card to make sure it is a valid card number. Then it proceeds to make the purchase. So it takes the $1 out, leaving you with only $49.00. This is not enough to purchase the $49.99 Package. So it will show Insufficient Funds. You must have $51.00 on an Itunes/Credit Card to make a $50.00 App Purchase.

Hope this helps.