I feel the rough cost should be promoted up front, I remember the last time you did one of these promo contests (or at least the last time one was on HN) I found it interesting, then was put off by the fact that you didn't allow viewing of the winning itinerary, and the fact that information on the service didn't seem to be as accessible as it could have been... I then completely forgot about you.
This time I clicked around, entered some information and saw the prices - and signed up to pay for a search. If the results are good, or even suggest promise for the future, I'll be sure to remember you. But I'm not sure I would have done any of this if a.) I wasn't sat here watching a football (soccer) match that's boring the hell out of me and b.) Next week I need to go on a trip to two cities in hopefully a single day, i.e. good timing
Edit: More feedback, would be nice to have more flexibility in requesting flight details. For example my trip next week, I don't care what day it's on (any of Mon-Fri would be fine), but I don't want to put +/- 3 days because I don't want one flight Monday and the next on Friday. Additionally I want to go to two cities, I don't care in which order. Maybe you could take a look at my notes on http://flightfox.com/contest/23238 and tell me a.) If it's OK that I specified them there and b.) If it might be possible to be able to specify them somewhere more efficient rather than just writing prose