American Express announced a new program yesterday that will let you pay your mortgage with a credit card.

It's only for cardholders with good credit and it's only got a single mortgage company you can use at this point---American Home Mortgage---but the feeling seems to be that this will expand to other lenders as well.

I was excited until I found out that it's only for new mortgages or refinances and that you pay $395 for the privilege as part of your closing costs. In other words, you can't start it with an existing mortgage, and even though you'll get big-time Membership Rewards points from sizable mortgage payments, it could be a while before it becomes much of a deal.

Think of it this way. In general, one rewards point is equal to one penny in cash back or merchandise. This isn't always true, but it's a reliable rule of thumb. In order to make up for the $395 you pay upfront, you'd need $39,500 in charges on your card. Here's your numbers to break even:

If your monthly mortgage payment is $658.33, it will take you five years to break even on that initial $395 fee.

If your monthly mortgage payment is $822.91, it will take you four years to break even on that initial $395.

If your monthly mortgage payment is $1097.22, it will take you three years to break even on that $395.

If your monthly mortgage payment is $1645.83, it will take you two years to break even on that initial $395.

If your monthly mortgage payment is $3291.66, it will take you a year to break even on that initial $395.

So, if you're going to stay in your home for a long time or you have a very large monthly mortgage payment, this program could make sense. But if you think you'll move again in the not-too-distant future, due to job change or moving up from starter home to bigger home, this isn't the great deal you might think.

And of course if you don't pay off your American Express card every month, then to put your mortgage payment on the card would be absolute lunacy.