What is the labour cost or time involved in building a wheel. I want to use a different hub with an existing 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.
I bought the parts for a wheel, was $65 for a new rim, $76 for 38 spokes (36+2spares), $10 for nipples and I had a NOS 7 speed hub already.
The bike shop offered to assemble the wheel for an additional $100 so roughly two-thirds the cost of the parts.
I spent a number of hours working on it myself and got it rideable, but it was not done well enough. In the end I got them to finish the true and that cost me $20.
All dollars are New Zealand and date from early 2017.
The wheel was a 36 spoke rear 700c road bike wheel, and my main failing was in tensioning and dishing.
If you do a front wheel, or a MTB wheel they tend to be a lot more forgiving than a road bike's rear wheel.
Answer: A third to half the cost of a handbuilt wheel is in the labour.
If you have time, and want to learn, its not hard to build a wheel. The art comes in truing and tensioning that wheel so it works reliably. HOWEVER If you're not mechanically minded, you're better off to just buy a new wheel or get the LBS to assemble yours for you.
The more work you do, the less they have to do so cheaper it is. Also, clean stuff for them, because that's nicer to work on. And allow plenty of time - customers who want it done immediately always pay the urgent rate, rather than "when you can fit it in" customers.
Depends on where you are an how well you want it done. Call your local bike repair shops and ask for quotes. They may also be able to refer you to specialized wheel builders.
Note that you will likely need new spokes as well as the new hub will require different lengths.
Depending on the quality of your wheel you may find it's cheaper and easier to replace the entire thing. Take the condition of the wheel rim into account as well.
I bought a hand built wheel set with custom parts for EUR 400 last year. Retail cost of all parts would have been EUR 533.
Especially spokes are cheaper in bulk and business-to-business. 48 Sapim spokes and DT nipples and washers alone are close to EUR 150 in retail now.
The labour cost of a wheel builder can be small compared to the increased retail prices we pay for some wheel components. Always check the market for custom built markets before buying parts and bringing it to your LBS. You may also get quotes from wheel builders with wheels similar to what you like to have.