Hey, my first thought is: you paid real money for that sand. Maybe post a local ad and recover some bucks? Someone doing concrete work will probably take it for a reasonable amount.
I have used low-grade sandy soil mixed with rotted wood in the bottom third of a barrel planter to create a kind of "moisture regulator" zone for my tomatoes.
But adding to compost? That could be tricky. What is the base soil in your growing area? If it's clay, then adding fine sand will turn it into a kind of concrete. Only very coarse sand (essentially small gravel) will help loosen clay soil. And if it's already sandy, I'll bet you don't need more.
However, if you have grassy pathways in your clay-soil garden, it's nice to add a little sand on top. That way you can tiptoe through the garden right after a rain and not track muck into the house.
That's my 2 cents' worth. Let me know if you paid too much.
