It seems that vendor-specific purchase prices are not tracked in system notes like sales prices are. Is this only possible via scripting? We would like to know the old value, who, when, etc.

Since the Vendor sublist is not editable for more fields, you would have to create a custom sublist attached to the the record and script to store changed values. Sounds more complicated than a spreadsheet.

Gotcha. Thank you. I'll just probably create a saved search of purchase transactions showing when the price is last used/first used.