Mass Update of vendor Purchase Price? Confusion

I go to "list > Search > Mass Update > Update Item Prices" and I get a field titled "Amount" at the top of the page.  I can put in a negative or positive amount or percentage for the price change.

I go to "list > Search > Mass Update > Update Purchase Prices" and I NO field titled "Amount" at the top of the page.  I can see how to filter the effected records ...but no way to enter the change that I want.

ALSO, for some strange reason, NS has several "vendor cost" fields for every item that we have created.  It has one in the inventory tab and one at the bottom of the basic tab. I am trying to change matrix items and I am trying to change the cost by the vendor listed at the bottom of the basic tab where it has multiple vendor pricing listed.  We have multiple vendors activated (but only have one vendor listed for 95% of items), can we mass update the vendor cost in the field that actually gets transfered to our drop-ship purchase