It depends on how large your yard sale is. We have medium sized sales and I'd do around $50 (a couple tens, 4 fives, 10 ones) and about $10 in coins (mostly quarters).
This sounds about right. I only get quarters for coins, because I only sell items in 25-cent increments. A few larger bills are important too, because I always have someone show up in the first 10 minutes of the sale and try to pay for 50 cents worth of stuff with a $20 bill.
We are having one tomorrow too. I'm starting with $50: two tens, three fives, and the rest ones. I figure if I need more I can ask DH to run to the store.