First of all, you should distinguish between price and value

The major difference between the two is that the price of Bitcoin may change even if the value of Bitcoin remains the same. This happens when, for example, the currency in which Bitcoin is priced changes its value itself. Further, if your accounting is done in some other currency than the US dollar, you should obviously use the Bitcoin prices denominated in your local currency, since it will be your base currency in which you will pay taxes and wages, a reference point of sorts. Consequently, you should price the goods you sell with the reference to that currency, and change the price accordingly. Many sellers which work with foreign currencies often increase the trade margins so as to avoid changing the price too often and not be subject to sudden changes in exchange rates