The B&K stuff isn't the most expensive nor is it the most accurate that can be found. I got a Hp 8656B for $250, and that is the best one to get. If you can find one for that price that works, it is real lab-grade gear, as they usually sell for twice that much.
But even the B&K will work because it has a calibrated output. The B&K will drift off-freq a bit, but for $150 it will do the job fine once it is warmed up and you modify it so that you can hook up a lab-grade freq counter to it.