It would be interesting to see a "shoot out" of integrated vs separates in, say, the $10k-$15k price range. This would need to take into consideration the TOTAL cost of ownership including appropriate cabling, etc. The additional cost of cabling could be an additional $1k-$3k, so it IS a factor in the total investment.
That being said, all things being (roughly) equal, the integrated has clear advantages in the bang-for-the-buck category. From a manufacturer's perspective, there are real costs that effect the ultimate sales price that have to be factored in.
Comparing ONE component vs TWO, here are some considerations: (In the case of mono amps, substitute THREE in place of TWO)
2 chassis
2 faceplates
2 on/off switches
2 sets of service literature
2 sets of packing materials
2 shipping cartons
2 items to warehouse/store
2 boxes to ship and handle
2 sets of promotional materials. Even if not in print, still needs to be developed separately
Etc.
I have separates. The justification is mostly intangible.