We wanted the limit to be high enough to where you wouldn't have to worry about hitting it unintentionally, but low enough to where you wouldn't get a crushingly hard credit card bill. We figured that $1,000 for a business was a safe balance between high enough and low enough. We also needed to account for customers who suddenly have a big burst in their traffic (which happens more often than I would have guessed). Believe it or not, we actually have many more tickets around people unexpectedly hitting it instead of people hitting it due to fraud.
The Top Up limit is only part of our auto-recharge fraud prevention. For international, we have very strict dollar limits you can spend (that you specify in our portal), a maximum outbound channel limit, and dedicated international gateways. So far this has effectively blocked all of the international fraud on our network and protected huge amounts of money with our customers.
For domestic traffic, you can specify concurrent channel limits in our portal that will SMS you if your channel counts suddenly spike.
Taken together, we are pretty proud of our success rate in keeping fraud down on our network