I saw a documentary about this machine when I was younger, the plan was to use the "Caspian Sea Monster” only to military purposes, as probably anything designed in the URSS back then, mainly for movement of soldiers and tanks across the waters.
As masklinn notes, the operating ceiling is 7500 meters (for which there is an efficiency hit). So long as the city is not too far inland, this outperforms air freight in all ways on paper.
Well, the thing has no landing gear, so the inland city better have a lake or something.
The 7500 meter figure, is that with cargo or without? Aircraft can typically do pretty amazing things with an empty cargo hold and a few sips of fuel in the tanks.
Honestly, I'm really having trouble seeing this thing as anything but a Soviet propaganda project with no real economic argument for it.
The thick, warm, rich air at sea level is not exactly the best operating environment for turbojets. And if there really was a benefit to harnessing the ground effect that outweighed the negative effects of low altitude, why not use a proper aircraft like an Antonov and just fly it low enough to take advantage?
And I assume the 50%/50% figure above is in relation to contemporary Soviet aircraft? Which where, themselves, thirsty compared to contemporary western aircraft let alone modern aircraft. The thing had 8 (Eight!) turbojets!
I think the implicit question is not "why aren't Soviet-era military jets competing with modern cargo planes?", it's "why isn't a modern version of this competing with modern cargo planes?". (The former is just comparing 1970s apples to 2010s oranges.) In this context, it's reasonable to assume a landing gear and modern engines.
Re: the Antonov, I think the idea is that it needs to be very low to utilize ground effect, and the only way to do this safely is over water with a hull capable of emergency water landings at the very least.
PS: Why did you answer your own question?