Keys are like 20 for 10 bucks on the Store, and I think you can get some from events, not sure though. You also get keys from subbing. I bought 1 key with the spare I had from a $10 purchase I bought 1 7 day 100% exp boost 1 7day 50% exp boost and 1 key, opened a chest and got a ruby that NPC'ed for 100 gold.