For every person denied boarding last year, eleven volunteers gave up their seat. I would bet they will be increasing the voluntary compensation to get more people to take it (and keep it less than the IDB they have to pay). Doing so will probably make the numbers plummet.

595 million passengers had tickets last year, 65,000 were involuntarialy denied boarding, or .11 percent (from the BTS)

I would not plan on there being that good of a chance for making money from IDB's