Well, I'd like to see the methodology for how five floors was determined to be the absolute lowest possible costs.
That's not really the end of the story though, anyway. As long as the price per square foot is still higher, it could make sense to keep building up. At a price of, say, $2k per square foot in Manhattan, if your costs are $500 per square foot for five storeys, vs. $1k for ten, the former earns you $(2000-500)*5 = $7500, while the latter earns $10k per square foot of land, so obviously it's better to build the latter.
That's not really the end of the story though, anyway. As long as the price per square foot is still higher, it could make sense to keep building up. At a price of, say, $2k per square foot in Manhattan, if your costs are $500 per square foot for five storeys, vs. $1k for ten, the former earns you $(2000-500)*5 = $7500, while the latter earns $10k per square foot of land, so obviously it's better to build the latter.