$50 maybe, maybe not. I don't know for sure. I used to get it done for $35 but that was like 10 years ago. Yes the strait edge and feeler gauges is the standard way of checking but I don't trust any strait edges that have been tossed around in the average garage. However the strait edges in a machine shop are usually treated carefully seeing their reputation is at stake based on the accuracy of their strait edges.
You are correct valves are not very common to fail on Nissans. However it depends on your definition of valve failure. In my opinion it would be a shame to replace a head gasket and just a few months or a year later learn that you have a heavy carbon build up or burnt valve when a simple inspection at the machine shop could have pointed that out to you. If you have the tools and knowledge you can check all that yourself even, but bottom line is I would check it or have it checked while the head is off.
It's much easier to travel through a maze when you have a map!
Former Nissan Tech
14yrs ASE all but AT it's a bitXX!
New career - HandyMan/Electrician - Freedom at last!!
86.5 Nissan HB D21 234K Miles
94 Nissan Altima GXE ? Miles
97 Dodge Grand Caravan 3.8L 132K Miles
64 Chrysler 300-K 2-door Convertible 63K Miles