With game pass core replacing XBL soon, these codes are being sold off dirt cheap. Each 3 month code will convert to 50 days of GPU for an existing subscriber, or 2 months for someone not currently subscribed. So a year works out around £43.70ish for existing subscribers.
You can only buy two codes in one order, but nothing stopping you ordering multiple times.
redeem here: https://account.microsoft.com/billing/redeem
for easy copy and pasting. Make sure you're logged in to the right microsoft account!
Max 3 year stack