Labour cost in building a wheel
It depends. If an inexperienced person such as your regular consumer does it, it could take a few hours especially if you don't have the proper tools or preparation. 

If an experienced person at the shop does it with their tools, it could take half an hour if all the spokes are the right length, it's a standard lacing, or they've done the same wheel before. For example, in this youtube video, a skilled wheelbuilder only takes 7:30 seconds  to built a new 32-spoke wheel up from scratch using pre-measured spokes. Dishing and tensioning the wheel would take perhaps another five to ten minutes. 


                
            

These days, many "hand built" wheels are really hand finished. The machine does the initial lacing and tensioning, the human does the final tension balancing. This could take less than five minutes of human time. 

Note that if you give your shop a random hub and a random rim, it'll take some time to research what spoke length you'll need for the lacing you want. Spokes might also have to be special ordered and/or cut to length. This type of research and prep work would add onto the actual time spent building the wheel. Rear wheels take more time in general than front wheels because of the dish. Disc brakes will also take more time because of the dish or if you change the lacing on that side. 

So I would say in terms of shop labor time, on the short side it'd be 0.5 hours of labor for a hub and rim that they're familiar with and have premeasured spokes for; to 2.0 hours for a hub and rim they've never seen and will have to do some research and maybe special ordering or fitting before they can start work on it. 

In the USA in 2017, shop labor is between US$40-80 an hour depending on location. This includes shop overhead such as rent and tool use. The shop may cut you a break depending on how much you are a regular. 

Tl;dr: Between 0.5 - 2.0 shop hours depending on how much research, special ordering, and preparation is needed as well as the complexity of the job.