Apparently there's a promotion for the new Xbox Games Pass Ultimate that lets you convert the entirety of your current Gold or Games Pass membership to Ultimate for just $1 with the current $1 for a month promo for Ultimate.
If you have (up to the max of 3 years of) Gold or Games Pass, you can convert it to Ultimate just by adding the Ultimate $1 trial month offer. If you have neither, you can get the $1 Gold deal, then get the $1 Ultimate deal and you end up with two months of Ultimate for $2. Ideally you stack up on cheap annual Gold memberships (maybe even using your points, see Brit's post two up), then add the $1 Ultimate trial. So buy low and, uh, game high? Hmmm.
The only catch is that Ultimate reverts to a $15/mo auto charge at the end of your subscription, so turn off auto-renew (and set a reminder to double check that it stayed off).
The pertinent info is here (the first two comments at the moment are a useful reminder to use Microsoft points and that this isn't a glitch but a promo):
https://www.gamespot.com/articles/e3-2019-how-to-get-three-years-of-xbox-game-pass-u/1100-6467749/