It is much more than just money. I have used external hosting for billing services more than once in the past. I lived to regret it. If anyone else physically owns your data, you are at their mercy. We were essentially blackmailed twice to get our data when we changed services. I will never let that happen again. Having said that, you also have to factor in the costs of backup systems. That would be a really bad place to try to pinch pennies.
