This is seriously way too high! If I had my way our guildies wouldn't pay a dime to use the store, but 25%?! Please please please lower this ZOS (lower both taxes - the game one and the guild one) - by putting such a high premium on the guild store you're actively discouraging people from using it!
Edit: Wanted a clearer idea of other people's thoughts on this, personally as a guild master to a merchant guild I'm none too pleased about this tax. Added in a poll to get a fairer idea of the consensus