I understand what you're trying to do. Unfortunately, attempting to do this with a drop down list will be quite difficult without some advanced scripting.
In my opinion, I think your case would be better served by using a simpler solution: You could use a numeric field with alternate display patterns.
Place a numeric field on the form. Select it and go to the Field tab in the Object Palette. In the Display Pattern property, enter the following pattern:
num{$Z,ZZZ,ZZ9.99}|null{'No Bid'}
This will have the effect of displaying the text "No Bid" inside the numeric field if no number has been entered into it. When a number is entered, it'll be formatted with the "$Z,ZZZ,ZZ9.99" numeric pattern.
Stefan
Adobe Systems