> One last thing: a new 5-digit ASN will cost you about $500 from ARIN, but there are auctions for 4-digit ASNs, and they run into mid-five-figures. If any of you can explain this to us, we'd be grateful.

To answer the authors question: lower number ASNs are seen as older more established networks when negotiating peering. It's the equivalent of doing a WHOIS on someone's personal domain and seeing when they started seriously internetting.

Obviously shorter numbers are more memorable and easier to type frequently, which is a desirable trait for network operators.

You probably spent a few bucks on "fly.io" and applied a mental value to the outward perception of a short memorable domain.