Pyros
<Silver Donator>
Depends on the shops I'd say, some sell the service directly, some only sell it as an addon when you buy a full PC like extended warranties and stuff. In the later it's usually cheaper/comes with a free OS installation but does force you to buy from only one shop. That said depending on what you're buying, the prices don't vary that much between shops, at least over here in Europe. Like I can't really find a top end GPU for cheaper by going to another shop, they all sell at roughly the same prices, so buying all the parts from one shop that will then put it together isn't a big loss, if any loss at all.I'm surprised these shops you mention will build your systems at all, unless you bought the individual parts from them in the first place.
That said while I've been buying PC pre assembled, next one I'll probably buy parts cause I want to make an open PC thing so I don't have to care about the air flow at all(saves a surprising amount of money if you don't have to buy 5 high quality fans and an AIO for your CPU too). I'm planning on putting it in the room next to my PC room so I have to look into extended cables and stuff too which will increase the costs but at the end the advantage should be 0 noise(well, in the room I'm in), good "cooling" and easy to modify stuff if needed(also easier to assemble since I won't have to give a shit about cabling and what not). Just need to remove dust every now and then which isn't a big deal. Waiting for these 7900 XT benchmarks and shit though.