I don't see you having much trouble with assembling the bike. The step that may give you trouble is adjusting the brakes. Getting the chain tension correct could be tricky the first time, assuming the chain is off the chainring (front gear) or cog (back gear).

Some bike shops are very diligent in bike assembly, opening the bearings, greasing them and adjusting them. Other shops just whip the bikes together as fast as they can. I just purchased a bike from Holland. The Manufacturer insisted that I have it assembled by a professional to preserve the warranty. It cost $95.00 because the bike was quite complicated. As well as the usual items, the bike had front and back racks and fenders. Putting everything together was time consuming, not difficult. Setting everything up so that nothing is rubbing can be fussy.

My guess is that assembly of a single speed would be about $50. If you have it half done and take it in to be finished, I can see them charging you for a basic tune-up. These cost about $50.

Good Luck.