I'd say that the monopoly mix is a good place to start:

2 x $100, 2 x $50, 6 x $20, 5 x $10, 5 x $5, 5 x $1 = $500 in 25 bills. Swapping one of the 20's for three fives and five ones is slightly thicker, but better for change.

Waterproof cash (ie coins) isn't a bad idea, either. Dollar coins and quarters might substitute for a couple of those dollar bills.

Here's another question: how do you package it? All in one place, or distributed a little around your bag. Beyond even the chance for theft, I'm a little worried that pulling out a roll of bills might tempt someone I'm haggling with to push a little harder.