While I have no strong gut feeling as to the total cost/benefit, as someone who has studied marketing and spent quite a bit of time in the sector, I get the impression the cost of advertising itself is very high, perhaps even dwarfing all the rest, comparatively.
And the industry as I've seen it is not particularly efficient/cost-effective or pleasant either. Most of the shoddiest work, unhappiest employees and shittiest bosses/manager that I've encountered were in this sector, by far. The only environment where I got a similar vibe was a bank/insurance company where I worked.