What should I do with my leftover bench flour?
The main problem I see is humidity - the bench flour can be lumpy, have bits of dough in it and will be partly moist. I would therefore not put it back in the main bag of flour. 

If you use it like flour, it might leave dry lumps, so it depends on your technique whether it will work or not.
But there are techniques in bread baking that use leftover dough or even dry ground and soaked bread in new loaves - that's where I'd put it. 

It can go into pre-ferments (even without yeast or starter), improving gluten development and dough texture. Unless it's absolutely dry, I recommend storing it in the freezer until your next baking day.

Apart from that: unless you are actively working the bench flour into your dough (vs. using just a bit to prevent sticking), have you considered using a "cheaper" flour?