Lets consider the price -
Say its all painted, all parts are there, nothing together.
Ream/sand tuner bushing holes, mount bushings, place tuners, drill tuner screw holes and attach tuners.
Sand neck pocket, mount neck - holes are there already.
Mount bridge.
Possibly sand string ferrule holes and press in ferrules
Mount pickups - possibly holes drilled for rings.
Mount pots and switches. Solder things up, adding capacitors.
Mount jack, probably more holes. Solder that up.
Mount pickguard, drill holes for that, install screws.
Drill holes for strap buttons, mount them.
String it up.
Do a trial setup.
Correct anything.
Fine tune the nut slots
Fine tune the saddle slots.
Do the final setup. Fine tune the pickup elevation.
Ya know, I can see spending a solid three hours on it, if its Warmoth stuff with a pre-installed nut. Give another half hour if he has to install the nut from scratch.
So, lets say he figured 3-1/2 hours work. Half a day. And thats for parts that he's not seen. That $300+ figure is looking more in line if the guy has a full time shop. Small private luthier... who's work is a full time second career, I'd still say $200 if he's serious about what he does.