The cost is based on your purchase history for the item, calculated from the beginning to date. It will  also include inventory adjustments if you use them.

It's weird to see it be negative/backwards if that is what you are seeing.  Not sure how you got there, but I'd focus on Credits and adjustment transactions to see if any of them look weird.

You might also Verify your company (File | Utilities | Verify) and if there are errors, review the results and see if any of them mention average cost for inventory items - specifically this one.  If so, Rebuilding your file (File | Utilities | Rebuild) will recalculate the average cost.