$500-$600 seems about fair.. I'd personally consider it. Mainly due to time constraints and the fact that there is a ton of stuff on my "to learn" list and building an MTB isn't at the top of it.
Just have all components sent to the builder, but then you also have additional shipping costs of what about $75? So that's $575 extra over total price paid for components in the build. Subtract $200 for tools, stand, etc, that you wouldn't have to buy and that's $375 over price of components for build. Personally, I think an extra $375 for a completed build by someone competent that knows what they're doing is pretty cheap. And a hell of a lot cheaper than the 1-4k mark-up of a similar name brand carbon bike. Plus I could avoid some inevitable damage that I'd cause to to my own lack of experience.
One thing I'd like is it the bike came pretty much ready to go. Tubeless tires, rim tape, sealant already installed. Tires and suspension aired up according to my weight. I don't own any pumps and do plan to get some eventually but considering the price of a bike isn't cheap, it's difficult to buy everything at once. If I could get rolling on my bike and be able to post pone buying pumps and sealant and crap for a couple of weeks - that'd be pretty convenient for me.