I work at a Lowes and we cull the lumber just before a new shipment.
At our store you will find a blue cart outside the contractor's entrance with a fixed price for the whole cart.
Granted, you may have to buy something you don't need, (store it if you have room - it will come in handy) but you will never get a better price on the lumber you do need provided it is there.
Don't be afraid to ask a manger for a discount on some poor quality wood.
10% is pretty average for split, warped, or loose/missing knots.
Now if they'd only cull some of that cedar.