An 8-pack of GL-KT-88s costs about $450. That works out to just under 10 cents/hour for a full rack of output tubes for an M-125-type listening pleasure.
I don't exactly molly-coddle my tubes, but I keep the bias in the .5-.55 range. No point I can see in running them up to .6 or higher. The amps stay in Class A and I think Gold Lion has made the penny drop in terms of sound quality. No doubt MontanaWay's fiddling with the the capacitors has helped, too, along with the delayed start relay Bob/Roy designed.
So for less than $1 a day, all day long, you get the finest sound imaginable. That's less than a single can of Rainier beer, as Longmire would say.