Don't go near this my friends. Threaten to revolt and give every cent of your flying activity to competing LCC's.
I've tried the bidding system with Virgin Australia. Some detail and experiences follow :
Bidding use to only be available to internet bookings - this recently changed to include all bookings.
Only the top bucket of each fare class is invited to bid for the next level.
Bidding is only for a cash upgrade.
I attempted a upgrade twice (on a 2 class flight) and did the math - what I'd paid for my Y seat, subtracted from the bottom J bucket, and bid the $ difference (minus $20, looking for a bargain, as you do). The bid failed.
Next trip, I offered exactly the difference between what I had paid for Y and the bottom J bucket. The bid failed.
On both these flights, J flew with 2 people in it.
I surmise that, unless Y becomes oversold, you low J bid will fail.
This means that their no such thing as a "cheapie upgrade".
If you were to bid the $$ to take your bid up to the value of the top J bucket you would probably be successful, but what's the point of doing this.