Buying ahead of time is your best bet.
Since you are talking two weeks with Keystone and Breck being on your list, the Epic Pass might be the ticket for you. It is a full season pass, but with lift tickets being around $100 day, it is easily worth it.
Colorado & Tahoe Season Ski Pass | Epic Local Pass | EpicPass.com
Of course I am not sure if you can buy it or not being international. You'll have to read the fine print.
Otherwise, buy your tickets ahead of time, or bag Colorado altogether. Utah, Washington, Idaho have much more reasonable by the day prices. A$pen is just damn expensive. Their multi day pass offerings are the way to go, cause you won't find cheaper.