You know, as I've been playing this game, I've been wondering why the three choices for contribution are set up in the manner they are.
100 coins = 50 contribution, 50 alliance points. Ok, that's fair, but 15 gems gets you WAY more than 100 coins. So why is it only 200 contribution and 200 alliance credits? and then 100 gems is worth 2000. If you pay attention to what you're spending, the best deal is 100 coins, hands down. It'll take longer but it costs you far less. A far more fair deal would be 15 gems = 2k contribution and 2k alliance credits, and 100 gems would be worth 20k of each. I mean, after all, as the alliance grows, the amount of value each one of those gives diminishes significantly, and each of those options is overpriced and gives very little to the alliance.
TLDR; If you guys could, I recommend lowering the cost of each contribution option, or raising the contribution each one of those options gives.

Second suggestion: allow speeding up the alliance upgrades?