The flights bidding obviously won't work if you are trying to bid like months ahead of your travel. I just ran your travel dates on kayak and the best price is $202 (including taxes/fees), and the return trip is even a non-stop flight. Your total costs are sure higher than direct purchase price. What was the lowest published price on 12/17? Your offer shouldn't be higher than 20% or 30% of the published price.
