I worked at a local PC shop for nearly 5 years, so I can give you a little advice from personal experience here.
A PC shop will most definitely charge you their full hourly rate to build the system. And, you might think that being a PC shop they can do it faster/better than you. However, that is not necessarily the case. Especially if you have parts that they don't normally work with (which is likely the case if you're building a performance PC). When I worked at a PC shop, I held the record time in assembling a standard PC, my time was 21 minutes (from raw parts to booting system). However, if someone ordered a specialty PC with unfamiliar parts it often took much longer (several hours in some cases), in part because more care was taken to prevent damage to the parts as replacements weren't readily available and also because we had to read the manuals or install guides on some parts to ensure they were installed correctly. They will most likely want to run through a Windows install as well, to ensure everything works. Then, there's the fact that although they built your system, they won't warranty it (they MIGHT give you a short warranty on their labor alone).
At the place I worked at, you'd pay $40 per hour. And, for building a PC out of parts brought to us, you'd be looking at between $80 and $160.
If you DO continue considering having a local place build your PC, get a signed agreement on how much it'll cost to build so they can't charge you anything beyond what you expect.
My advice is to use the tutorials others have posted and take your time and build your own system. It's not that hard, it's a fun experience, and it'll make you much more familiar with your PC's insides...all that and the right to brag about it. LOL