Any reason why I get [object object] as a result? Ideally, it should set the overall average cost from the item record in a custom transaction column.

search.lookupfields returns an object

Because lookupFields returns

Object | Object[]
* Returns select fields as an object with value and text properties.
* Returns multiselect fields as an array of object with value:text pairs.

https://docs.oracle.com/en/cloud/saas/netsuite/ns-online-help/section_4345776651.html

Perfect! Thank you!