I guess it depends if you want to match grain, etc. When I toured the Stickley factory a couple years ago, the guy showing us around said on a good day they'd net out at about 50%.
Sharp solves all manner of problems.
As previously stated, it depends.
For man made sheets, usually is close to zero the surplus I purchase. Quite close to zero for very high quality for prepared and squared lumber when I choose each piece carefully.
For rough lumber it can be 10% up to 50% depending of the required and available sizes and the wood quality.