Getting a number you like (especially if it's 4-digit) vs random pick by the RIR.

According to their previous sales page, 5-digit ASN's have been selling fairly fast at ~$2,000 level even though as you point out you can get one for just the RIR registration fee (ARIN at least will give you a 6-digit by defualt but 5-digit if you ask when applying).