Completely false. The transportation and logistical requirement alone would have outweighed the costs of a mass grave or pyre.
The cost of a mass grave is that now the entire city knows you're killing people and you're coming for them. It leads to formation of resistance forces, kills your soldiers, stops the occupation from progressing.
Transportation is free because you took over the railways. Building concentration camps is free because it's done by literal slaves who have no choice anyway, and is done with local materials taken from existing housing. It's not like Nazis were paying market prices for any of this.
Like, I have no idea why were even arguing about this. Go and visit Auschwitz, it's all explained in the museum there. Read some books about occupations of Lvov or Warsaw - Nazis very quickly stopped executing people en-masse because it was more trouble than it was worth. You can say you disagree, but that's just arguing with facts.
"China isn't committing a genocide like the Nazis, because they could just shoot everyone. But, the Nazis didn't do it either, so the Nazis weren't committing genocide like those damn Nazis either"
Bizzare argument.