It appears that model page edits or markup page edits can yield strange values in the PRICE column of the downloaded CSV. I had made some changes to some models via their edit mode pages which resulted (in the subsequently downloaded CSV) in zeros or negative values for the prices. The negative PRICE values were the same as the correct MARKUP values, only negative. I then found the markup editor page via the forum (http://www.shapeways.com/myshop/markup) and made changes to some of my items while reordering all of them. Immediately afterward I downloaded the updated CSV which now shows ALL negative versions of the MARKUP values within the PRICING column. Still the MARKUP column is fine.
The same incorrect price values show when importing into both OpenOffice and Google Docs.
This seems to defeat the purpose of having a CSV for markup changes in the first place, as it's necessary to see Shapeways' prices in order to decide on a reasonable markup value.