The more you pay…
Brendan Tompkins' chart showing the correlation of bugs to software cost is probably not far off.
This is because cheap software is often cheap thanks to economies of scale - meaning hundreds of thousands (or maybe millions) of users.
When you've got 10 clients, having 10% of your users calling you isn't so bad; when you've got a million clients, you don't want your phone ringing 100,000 times.