Always explain fees associated with jumps canceled because of inclement weather. Never obligate yourself to jump in 30 knot winds or from 1500 feet, just below a cloud layer. It's just not professional, it's stupid. To jump in marginal conditions, because you haven't got paid, is just poor planning. Remember that you have no control over the weather.
Many professionals consider contingency fees to be unscrupulous.
Good demos are arranged months in advance. You have expenses regardless of the weather on the day of the demo. If you think this is crazy, find out how much your local DZ pilots get for showing up on obviously rained-out days. Local demos, that your DZO tells you about, generally have you pay for your slot or sometimes you get the jump for free. You should get paid for these demos too.