Just making sure, there is a product object and variants object surrounding the Inventory Item correct? I've uploaded the majority of the mapping that I used so you can see the full hierarchy.
If your hierarchy looks like mine then enable debugging on the import to Shopify step and see if it gives any feedback. It should give you some sort of error in there if it is rejecting your value based on format. I hardcoded 10 as a price and sent a matrix item with 3 child products over using the setup in the image and it went into the Cost per item column cleanly.
