That's effectively a 50 percent discount. While both Sprint and T-Mobile are offering 50 percent discounts on the Note 9 (the latter with a trade-in), both require you to stay with them for a long time. Sprint would give you the discount on a 18-month lease, while T-Mobile will reimburse $20 from your invoices over a 24-month period until it gets to the $500. The Costco offer, however, is totally commitment-free.
The list of trade-in phones is long and includes phones from 2016 that are low end compared to the Note 9: Galaxy Note 8, Galaxy Note 5, Galaxy S8, Galaxy S8 Active, Galaxy S8+, Galaxy S7, Galaxy S7 Active, Galaxy S7 Edge, Galaxy S7 Duos, and Galaxy S7 Edge Duos.
Even if you don't have a Galaxy S7 lying around, remember that you can get one used for around $230. It may be worth buying one and trading it in to get a contract-free Note 9 with an effective $270 discount.
If your Samsung is not in that list, there is another set of phones that will knock off $250 off the Note 9 price without a contract: the Galaxy S6, Galaxy S6 Edge, Galaxy S6 Active, Galaxy S6 Edge Plus, Galaxy Note 4, Galaxy S5, Galaxy S5 Neo, Galaxy S5 Prime, Galaxy S5 Active, Galaxy S5 Duos, and Galaxy S5 Sport. The latter phones were released in 2014.
Note that you need to be a member of Costco to get advantage of this offer and it only applies for Note 9s bought at the company's warehouses. After buying the Note 9 and trading in your old Samsung, you’ll get a rebate in the form of one-time bill credit and a prepaid Mastercard, which should arrive from two to three months after your purchase.
It's not as good as an instant discount but, hey, $500 off a $1,000 flagship phone and no contract? Definitely a good deal.