Armstrong feeler gage sets are like between 30 and 50 dollars for short or long bladed sets. Proto isn't that much different price wise either. SKF sets for long bladed are nicer though in my opinion because you get more sizes in the set. I buy Armstrong because I guess that is what the local place sells and I'm lazy. SOME DAY I will corner an SKF guy and get some schwag though.
That being said they are not rugged enough to be considered something that isn't a disposable item, imho. I replace my long bladed set probably every couple years, give or take? Depends on how much I actively decide to use them I guess. I would say that the more on the ball millwrights I work with replace somewhere in that time frame if they are using them a lot. Depends on what they are being exposed to though, but like chlorine rich environments eat the hell out of them it seems. (FYI, NEVER spray that rust remover spray on them it will ruin your feeler gages.) They get wet, they get rough, pinched from sliding into places, etc.