Well, back in the day, a doughnut cost a lot less than a donut.

So, betting dollars against donuts would be a bad idea.

Not that it was ever a betting term, it just condone contains a reference to betting.

It's like saying "hey, lets get dinner. You give me a twig for every dollar I put in."

You're either exchanging something of unequal value, or making an "investment" where you get very little return on it.