Well.... I can't speak for the big time carb tuners out there, who are in a totally different league than a hobbiest like myself. I'm gone 6-8 months a year so obviously my experience is stunted due to this.
But I can buy a 750 dbl pumper for $100-$125
Strip it, make metering adjustable, rebuild, redye, and replate the carb so it looks like new.
Test it on an engine for any weirdness, go for a ride with the O2, make a few adjustments, and sell the carb for $400-$500 which is pretty close to what a brand new "mystery calibration" carb would cost.
It's far more depressing to have to do all this work to a carb that cost $600 new!
This is why I'm stockpiling old style Holley's with any money I make with this hobby.
I dunno, if it was me, I'd find someone willing to do that sorta work local to you and tune it on the engine.