That's only a starting point. Money spent = a kind of 'cost' function, with 'cost' representing raw materials, energy etc used to produce something. Higher cost = more resources used / more damage to environment.

But that's only accurate if all costs (including environmental) are somehow accounted for in market price(s).

That is rarely the case. Apart from the difficulty of assigning $$ values to things like tropical forest cut down, mountain top removed to get copper ore, water used & polluted, or what have you.

Also it doesn't include social costs. If product is cheap but someone's health suffered, or profit helped dictator to reduce press freedom in producing country, is product still cheap?

Too many unknowns & externalities - $$ spent is a poor indicator.