The prices are updated daily but rounded to the closest .99 as it means the prices don't actually change daily and confuse people but do stay very closely in synch and at round numbers people can understand. The exchange rate is set by the banking system. We have no input over it at all despite what you may think
Every day we ping the payment providers server and grab the latest prices and update those in our database. You can choose whether you believe us or not, there is nothing we can do about that, but this is the way it works - honest!