Ideally, someone in the era prior to yours has a treasury building though and in this way they cover your costs.

What the treasury building won’t do for you is donate the previous era goods. This means you’re costing the guild whether you’re a major GE competitor or not. Whether or not you go beyond Tier I is irrelevant, the guild has to pay for the potential of every member to complete every unlocked tier. Having a treasury building means you can help cover most of the cost for your participation in GE. This is why many guilds push for all players to get a treasury building and level it a bit, even if they aren’t a GvG guild.

To do GE though, every week, there are costs associated to the guild itself, based on the age of all the players Tuesday morning when it opens. It’s a function that will really show you where your city is lacking, and where your strengths and weaknesses are. Guild Expedition quickly won the hearts of many players, even if it didn’t replace their love for GvG.

Forge of empires guild expedition guide