[What he does first before answer is check the price per whatever-unit-of-measure-is-used-here-probably-metres (either way, it's interesting to think of how unit of measures work. They do use different ones in different countries...perhaps when the Silk Roads open properly they should develop a more standardized unit of measure).
Only when he saw the price did he answer (because it was important).]
no subject
Only when he saw the price did he answer (because it was important).]
I think it's fine.