But if it was tripod-mounted or used on a plane-table for precision work (which also automatically held it perfectly level), one should be able to read it down to 1/5 of a degree. That's about the limit of accuracy for setting the regional magnetic declination offset as well.
Not sure what the inflation calculators say, but I'm looking right now at the original Brunton box it came in on the book shelf above me, and the original price sticker on it says $150.30. That would have been around 1982-83.
It's quite a beautiful precision instrument.