Despite dimecoin price changing constantly, my 'est btc' balance stays exactly the same. How often does this exchange update prices?
You misinterpret "est btc" - this means: "You will have so much BTC if you can sell for BTC"
The price at which coins are traded for BTC is 0.00000001 - and balance is calculated at this price.
Similarly incorrectly CoinExchange and Nova counts