Hi Taulukos,
Thanks for letting us know about this. I just tested this behavior in one of our test accounts and was able to successfully save an inventory item with "84.00" entered in the cost field, so I'm not quite sure what may be causing the "Invalid Unit Price" error to be returned when you attempt to do so.
If you are still experiencing this problem, please contact our customer support team by completing the web form on the following page of our site: http://www.auctiva.com/help/requesthelp.aspx
-Mike