We need 100,000 gold, at the time I'm writing this we have 107,103 gold so we have enough to upgrade.
The only things left to buy after getting to 5th guild level is upgrading the guildstone, 100,000 gold and 10,000 GP to get the 'guildstone of hope', a larger guildstone with silver detail, we could also get the 'guildstone of courage' which is 1,000,000 gold and 100,000 GP.