In my case it is really lay over time that plays a role. Often the cheapest flight isn't a direct one and this means time at an airport where one uses the time to work and catch up on emails etc. a good lounge would really come handy here.
Even with priorty pass that isn't an option at many airports because they are either in the wrong terminal or there are none available in the program at all for this airport.
However most of you are probaly right about the cost factor. An alternative I find interesting are those terminal hotels rooms / bubble rooms you can rent by the hour. It gives one a bit of quite and in some cases a shower and internet to work with.
Some airports are experimenting with them near concources - not a bad idea for longer lay overs.