I've gotten $1100 travel voucher from Delta one time, just had to wait seven hours for the next flight. It was worth it though because I knew I could use that voucher towards my tickets to London last year for the Bills/Jags game.
Delta usually starts at $200 and goes up in $100 increments. You've just got to be willing and able to change your plans, and hold out for anything over $700. Anything less is not worth it.