I'd say if you put in the minimum bid for all flights the chances are fairly good you'll get an upgrade. Not that many people bid for upgrades and the prices are fairly expensive for what is a very average product.
That was my initial inclination, but then I looked at the predictability meter and the minimum bids, which are like $240 for transatlantic and $150 for intra-Europe are in the red zones, which seems to indicate the chances are nil/red, not just "low?"
The flights seem very open, as I can book at least 9 seats in Saga on each of them. I'd love to modify the nids to the minimums. Does anyone know anyone who's bid the minimum? To me, the minimum should mean "minimum acceptable bid unless you get out bid by others," which is where I agree with your comment that there probably aren't that many upgrade bidders, and the minimum, or minimum plus $10, should suffice???