[smyles]

Combox is combox, there're 2 or 3 flavors, but all are model independent.

BMW is retiring old cell tech and offers free replacement or $$/credits for current Assist subscribers.

And not to bash BR or any other coding/retrofit vendors, but what they charge you is knowledge/know-how that you could gain yourself, plus convenience and some kind of guarantee. It would take you quite some time to search, read, learn and practice, so it's up to person to decide whether what's their time worth to them. In terms of parts, in your case besides the combox itself I see ~$60 for main harness (which, again, is not necessary, but offers convenience), few bucks for couple of cables and ~$20 for the coding cable.