What's the cheapest way to set up a conference call?

It's a question of whether you want toll free access or not. If having a toll free number is not that important, then by far, the cheapest way to go is Flat Rate.

If toll free is important, then you are better off with the SuperSaver plan which works like a cell phone plan -- you buy a set number of minutes each month at a discounted rate.
Remember, most residential and business phones have free long distance bundled with their local service. So having a toll free access line isn't as important as it used to be. People can usually dial anywhere in the United States with incurring any charges.