It's not that they've paid this much for him, they've had success investing a lot in young players, it's that they've done it so late. It reeks of panic buying.
A deal was always difficult to do, he extended his contract earlier in the summer. It was only when Monaco were knocked out of the CL qualifiers last week did they agree to negotiate. If they had gone through Martial would most likely have continued to be a Monaco player this season.