Pierre,
I realize that this is an old thread but since I stumbled upon it, and I have often learned alot about Moodle from old threads, I thought that I would add a few points.
First, I now do as you suggest above: I now check the values of wild cards that represent prices to ensure that they don't finish in 0. For example, I edit any number such as 8.30 and change it to 8.35 or change 10.70 to 10.75 etc. (BTW, it's not that laborious for anyone who does accounting.... but alot of my prices finish with a nickel or with a 5!)
Second, it took me time to understand that I can change/edit the value of a "wild card" after Moodle has first inserted a specific value. ("Preset" might be a better term/definition rather than "wild card" - since the values are not really random but can be edited later.) That is to say: I now recommend to colleagues to add only one value to a wild card on first creating a calculated question, and then add more values later.
Anyway, thanks for your various posts Pierre.