Heres the link on how to do it for less than $5/mo.
I'm in the process of doing this now:
Process for Costco members
(Note: rhutvirani has pointed out an expired costco membership worked, so YMMV.)
This assumes you have no active memberships with Microsoft. If you do, reduce the amount you by accordingly as the maximum is 36 months.
1. From costco.ca buy: (don't forget to use the code AERO3 for Aeroplan Miles)
two 12 Month Xbox Live Gold Membership – Digital download Item #1060292 for $60 each
one 6 Month Xbox Live Gold Membership – Digital download Item #1145546 for $38
2. You will receive codes in your email. Got to https://microsoft.com/redeem
and apply one code at a time. Each time it will give you the option of getting 1 bonus month in exchange for setting up auto-renewal - do it all 3 times. This auto renewal will vanish anyway in a few minutes as you migrate. You have now added 12 + 12 + 6 + 1 + 1 + 1 = 33 months of Xbox Live Gold to your account.
3. Now go to HERE
to subscribe to PC Game Pass for $1 for the first month (additional months are $6/month but that doesn't matter - it'll get cancelled in a few minutes in the migration). You now have 33 months of Xbox Live Gold to your account and 1 month of PC Game Pass on your account.
4. Now go to HERE
to subscribe to Xbox Game Pass Ultimate for $1 for the first month and migrate everything else over to Xbox Game Pass Ultimate. You'll now have 35 months of Xbox Game Pass Ultimate.
5. Now go to https://account.microsoft.com/services/
and turn off the recurring payments of $17/month just so you don't forget in 35 months time to do it. Note: you do not need to cancel the Xbox Live Gold and PC Game Pass; the migration process in step 3 has closed out those services already. The only recurring payment to stop is the one for Xbox Game Pass Ultimate.
6. You should see on that page the following:-
"Xbox Game Pass Ultimate"
"Expires on 5/xxx/2022" (IE May 2022 - 35 months from now)
"Turn on recurring billing" as a clickable link; which indicates it's not on
Edit: Works. 35 months Xbox Game Pass Ultimate.