Money is more fungible and can be used to buy goods that aren't on sale. The question here is if Reimu is fast enough to collect it before the opportunity cost becomes too high... she doesn't have Marisa's speed or collection radius. It comes down to how picky Reimu is... if she can still manage to spend her money on satisfactory items after this diversion then she comes out ahead. If this diversion gets Sanae something that she must have, at a sale price lower than 100 yen, then it's win-win.
