It's also worth pointing out that the average price of steem calculated in dollars is based off the Bitcoin price as well. So if you see your payout dropping it is likely due to either the steem price dropping or alternatively the actual Bitcoin price dropping as well because many alt cryptocurrencies are pegged to the value of Bitcoin.

So for instance because of poloniex's trouble and so on with the steem price bumping up and down I've seen my payout change quite a bit and that happened with the bitcoin price dropping as well but since then both have gone back up and my payout is almost back to where it was when I first got the upvotes.