I'm dealing with a similar issue -- or at least I'm having a similar issue
trying to reverse engineer the cascading list examples. I'm actually trying
to populate a list box based on the selection made from a dropdown. I've read
the cascading list box help section, I've played around with the filtering,
but I still can't get it to work. I feel like I'm close, but close doesn't
count :)  And after working on it all day, I have something like an ice cream
headache, only without the fun.

What I am trying to do is this:
a) Created a dropdown list for Items, which is linked from a SharePoint list
for catalog items (item column);
b) I created a list box for ItemCost, which is also linked to the SharePoint
list for catalog items (cost column).

I want the cost for an item to display in the list box based on the user's
selection. For example: User wants to order a coffee mug, selects coffee mug,
and voila! ...the cost appears in the Cost field.

In the list box properties screen for the Cost list box, under Entries, I
have selected the "Select XPath" button and then the filtering option. The
filter options available don't seem to correlate with what I'm trying to
achieve. Is there some way to write an "If then else" here? What's the best
way to go about it? Do I need to code some XML statement to achieve this?