What research has examined the cost in time and money of conference attendance?
I published a paper:
 Anderson, L., &amp; Anderson, T. (2009). Online professional development conferences: An effective, economical and eco-friendly option  Canadian Journal of Learning  Technology, 35(2). 

in which we calculated the costs (transportation, hotels, meals etc.) in dollars and in Carbon costs using a medium sized conference in the UK.
We were interested in showing the HUGE cost and carbon savings if the conference was held online and used this data in our book on Virtual Conferences.