@id properties are used as identifiers for compatibility with JSON-LD. The value of such a property must always be an absolute URI that provides a stable globally unique identifier for the resource, as described in RFC3986.
The primary purpose of the URI format in this context is to provide natural namespacing for the identifier. Hence, the URI itself may not resolve to a valid endpoint, but must use a domain name controlled by the resource owner (the organisation responsible for the OpenActive open data feed).
Whether there are any additional entitlements (other than those listed in entitlement) or other types of discounts are associated with the Customer Account that will influence pricing, and therefore whether the pricing for the entitlement in the feed should be treated as indicative.