My guess is that if enough people respond the offset book will cost less per copy than a POD book. If not enough people respond to make that possible and if the book were necessarily going to be an expensive one then of course I wouldn't do it.
As Joel said, why worry about the cost until we have cost figures. In the meantime, do go forward with other alternatives, if someone has the energy and time to do that in a serious way.
Michael A. Smith