This is definitely what the market will bear kind of thing, and it depends also on how thorough the assembly is.
If you do this, as a pro mechanic, PLEASE KEEP IT IN THE ORIGINAL BOX. Your preassembly does NOT help--it just means more to check. Mechanics have a routine on new builds and will be faster and more consistant if they can just keep to it, rather than checking to make sure nothing was done wrong.
I personally try to match actual build time to cost, since some bikes really do come together faster than others. Canyons, for example, really are more attaching a wheel, straightening the bars, and then checking over what is usually a good assembly. Bikesdirect is pretty similar to how a major brand ships its bikes to shops--needing rather significant assembly and adjustment, even if everything is mostly where it belongs in the box.
Something like an hour at the shop's labor rate is probably around average.