Asset Register - minimum value of an item to record

In the context of ISO 27001, the purchase value of an asset is irrelevant - according to the standard, you should list all information and information systems related to your information.

For example, a USB drive where you backed up all of your sensitive data might cost $5, but the value of this data might be huge for a company. 

See also this article: How to handle Asset register (Asset inventory) according to ISO 27001 https://advisera.com/27001academy/knowledgebase/how-to-handle-asset-register-asset-inventory-according-to-iso-27001/

These materials will also help you regarding handling assets:
- Book Secure & Simple: A Small-Business Guide to Implementing ISO 27001 On Your
Own https://advisera.com/books/secure-and-simple-a-small-business-guide-to-implementing-iso-27001-on-your-own/
- Free online training ISO 27001 Foundations Course
https://advisera.com/training/iso-27001-foundations-course/