I don't think that expense has been the problem, rather the efficiency has been the problem. Most of these machines only affect a very small area around the mac

I don't think that expense has been the problem, rather the efficiency has been the problem. Most of these machines only affect a very small area around the machine itself, even with the inclusion of fans to increase the area reached it is still so small that it requires lots and lots of machines in a public area to make a difference. The problem then is the space required to place the machines, e.g in urban areas rather than the cost.