Charging more _not_ to write tests

Joel Clermont replied to a previous email on Twitter:

I once toyed around with the idea of charging MORE to do it without tests. Had a client pushing back very hard when I mentioned testing (what I thought they'd perceive as a benefit). Ended up not pursuing it, but was a fun idea at the time.

It's an interesting idea.

How much more would you charge not to write tests?

How much more would make you feel comfortable and confident that the application works as expected, and would cover the additional time for finding and fixing the bugs caused by not writing tests?

As Joel said, it was a fun idea, though it seems counter-intuitive to me, as the a client would be paying more for worse software.