Adobe Acrobat 9: Combo boxes and pre-populated fields

Question

situation: I'm trying to implement a pretty basic combo-box form driven pdf file.

I have a combo box (CB1)that has these items:

Item 1 Item 2 Item 3

I would like it so that when Item 1 is selected, it changes a form field (Total) which reflects it's price.

For example, user clicks on combo box and selects item 1 and on the Total field, it shows up as 100. If they select item 2, it shows 200 etc.

What is the simplest way of implementing this in Adobe acrobat 9.

I've looked at some examples and found this to be the closest: http://acrobatusers.com/tutorials/2006/change_another_field

but the code seems to be a bit difficult to understand.

thanks,