No, with a sale you just get the mark-up on the part (remember, we have to BUY the parts).
Ah yes, fair point. Obviously not all LBS are dishonest, but the advice given to the typical customer wouldn't be, "you could rebuild this yourself" (most people are not mustachian, or skilled in bike repair) so the LBS would tend to offer their services, after all why did you take your bike to them in the first place?
The option of DIY rebuild wouldn't appeal to non-mustachian types so you would be unlikely to hear about it from your typical mechanic (who is out to make a profit, after all). The first time I had a broken spoke the mechanic at my LBS took £20 to replace a spoke true the wheel. At the time I had no idea how simple a process it was to replace the spoke. 4 weeks later I had another broken spoke, so £20 down the pan! I bought a cheap replacement wheel to save me taking the bus, I did my homework and rebuilt my wheel when I had time and have a nice spare waiting for the next time something goes wrong.
Economical and ecological motivations for DIY repair are very different beasts, so yes it may not be good financial sense to spend 10 hours rebuilding a cheapo wheel, but mother earth will thank you for not throwing away a perfectly good hub (and possibly a rim too).
AS MMM covers pretty well in a number of posts, benefits to the DIY/insourcing approach go beyond good financial sense. You learnt fix your bike yourself, so if you have a problem, you need not worry about hauling your bike to the LBS. The second time you build a set of wheels you can do it in 2 hours, not 10. You can build a set of wheels for your friends for a tidy profit. You can upgrade your cheapo tyres with fancy pants custom build bombproof wheels at component cost price etc etc.
Of course, if you get zero pleasure out of doing this kind of repair, maybe outsourcing is for you. Personally, I prefer the feeling of "
I fixed this, it may well have cost me more in time and tools than another cheap wheel, but the next time it goes wrong, I'll know what to do and still have the tools". Only last week did I stop to help a fellow cyclist who had a broken spoke, I told them exactly what was wrong and what would happen if they got their spoke replaced. I gave them my e-mail and offered to rebuild their wheel. Side hustle story in the making, lets see if they get in touch...