As a lot of you guys have pointed out, Costco isn't actually selling the GamePass membership, this is a conversion deal. I bought the Xbox Live cards from Costco b/c I was more comfortable doing that then going with G2A or CDKeys or whatever, but you can save a few extra bucks going that route as well.
Also, this isn't open to existing GamePass subscribers. It looks like you need to be a new customer to get the $1 offer.
Finally, you MUST load all 36 months of Xbox Live to your account before upgrading. The "trick" is that the upgrade will retro up to 3 years. If you upgrade and then add Xbox Live, it's going to convert $ for $ instead of month for month. So 3 years at $5/mo. = 1 year at $15/mo.
Sorry for the confusion, I'm brand new to the Xbox world and some of this was probably common knowledge.***
Microsoft has an offer now- $1 for your first month of Game Pass Ultimate (includes a bunch of games, including Kingdom Hearts 3, Forza Horizon 4, Minecraft, Halo, etc.). They'll also convert up to three years of Xbox Live Gold Membership into Game Pass Ultimate when you upgrade. There's some reference to "conversion rate", but 36 moths = 36 months.
Right now, Costco is offering 2 years of Xbox Live Gold for $95, and 1 year for $55. If you buy both for $150, then add the $1 trial of GamePass, you will end up with 3 years of GamePass Ultimate for $151. IMO this is a pretty good deal- GamePass Ultimate is regularly $15/mo.
I just got one of those refurbed Xbox One X's today, and signed up all of this this afternoon. It worked flawlessly- I have about 30 games queued to download overnight.