I have limited experience in this area (I have successfully bid on domestic trips twice and unsuccessfully bid on international trips a few times), but my hunch is this:

The only way you are going to save big bucks by bidding is if you are in the situation where you have to buy an international ticket on very short notice
and don't want to pay the last-minute "gouge rate" that airlines charge for this. If you are in that situation and an airline has a flight with a lot of open seats, you might be able to get one for approximately the same as the advance purchase fare. Again, this is just my hunch, but I doubt that anybody ever gets a deal on Priceline for an international ticket that is very much less than the standard 14 or 21 day advance purchase fare. When you factor in the loss of frequent flyer miles (depending on your status with the airline and the length of the trip, these alone could be worth several hundred dollars), losing the ability to choose your departure time, connecting city, etc., bidding just doesn't make that much sense to me.