Excellent deal for those who are existing subscribers. CDKeys offers 100-Day of Game Pass Ultimate via Xbox Live Gold Conversion for $17.49 when you follow the instructions below. Note, this offer is for Current/Existing Game Pass Ultimate Members only.
Deal Instructions:
Purchase 6-Month Xbox Live Gold Membership $17.49
Note: You will receive 2 codes of 3-Month Xbox Live Gold Membership. You will be prompted to extend your Game Pass Ultimate by 50 days for each code (100 Total). The renewal date should be the same as your Xbox Game Pass Ultimate plus 100 days (Maximum 36 months).
Credit to SlickDeals for the original post. I've tried this successfully, just remember not to stack the codes beyond 36 months.
Unusual these days to get a good deal for existing subs… Cheers OP