@sirrian - This gem cost for GW Sentinels is, in my opinion, going to be the most divisive decision the devs have yet made (at least in GW) and here's why, if I'm understanding this correctly, this is a huge mistake on your part:
1) You're asking players to contribute 340 gems each week with no assurances that anyone else in the guild is also going to contribute
2) Not only that but this is forcing players to gamble with their resources. Forcing them to gamble that, not only will other members also contribute but that the other members are going to perform at such a level as to make risking these resources worth it.
3) On top of that, since the Devs will be personally curating the fights (for which guilds fight which other guilds), the Devs will basically have it in your power to decide which of the guilds win each week (i.e. if one guild in the bracket is dominate then having them show up in your bracket for the week basically means it's not worth spending the resources since your not going to win for the week anyway).
4) This will further alienate the casual guilds and players from the hardcore, destroy the semi-casual middle ground and make it much harder for players to find a playstyle that suits them (and a guild that fits it).
Please reconsider how much you are asking players to contribute. I can't speak for other guilds but personally, I'd be fine spending 50-100 gems total a week to max out all my Sentinels and I'd be ok with asking my members to do the same as they'd still have 4/5's of the Guild Task Rewards left over. Any more than that and I'm probably not going to sign my guild up for Guild Wars as it's just not worth the risk regardless of what the potential rewards are.
Edit: I wanted to add @Mithran's point as well as it's one I initially missed and also very valid: