Purchase a 3-month Gold membership and receive a code for an additional 3 months delivered to your email address within 10 business days.
If you turn on/off auto renewal for each code u can get an additional +1 month to your subscription per code, so 4 months per code.
There's a cb site that offers $5 back on 3 month purchases of XBL Gold, so technically $19.99 for 8 months.
Yes I'm aware the best way to get XBL is free thru Microsoft Rewards lol.
And that this is not as good as the previous $14.99 sale for XBL Gold https://slickdeals.net/f/11192091-buy-1-xbox-live-gold-3-month-card-for-14-99-get-a-token-for-a-free-second-card-at-a-later-date-ymmv-xbox-com