I would much rather have a free, no-mileage roundtrip ticket than a $-off voucher.
I wouldn't even consider volunteering on a Delta flight unless the offer is at least $600.
In terms of miles earned on the free trip, the best case scenario for a $200 voucher is a 4,000 mile roundtrip with a 100% Gold/Platinum bonus. If you value 8,000 SkyMiles at 2 cents each, your $200 voucher is worth $360 to you. I can find plenty of itineraries that cost more than $360 roundtrip.
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"It's as easy as 1, 2, C" -- Kelly, Married With Children
[This message has been edited by JS (edited Dec 01, 2003).]