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You can check here to have a glimpse of the market: https://www.coingecko.com/en/price_charts/steem-dollars/usd
You can check also steem and other crypto currencies. Imagine that couples of weeks ago 1sbd was 17 $! :)

Thanks but there is something that I don't quite understand. If I do the math in my wallet the total value converted in usd should be higher

I asked a good friend, @gric to help you. You will have a comment in 10-15 minutes. I am not an expert in these kind of things as well. 😂

You are really kind thanks. Tell him to take his time because I'm going out so there is no rush ;-)

SBD was initially pegged to the US Dollar but this changed in November 2017 after it was listed on a Korean (I think) exchange. Since the circulating supply is currently limited to just 8,540,104 SBD many investors probably think it might be a good idea to buy into it. So what you see on Coinmarketcap is correct, the price even spiked over 15 USD at some point.
The total value of your wallet is now approx. 267.6 USD. Steemit is calculating the value based on an average of 3.5 days (I think) and since SBD in now rising you still see the lower value in your wallet. If you want to see the actual value of your account you can install the Chrome addon SteemPlus. This will also add some new functionality to your Browser for Steemit.

Thank you very much, now I understand :-)

You're absolutely correct 1 SBD is equal to $4.78 when I comment this here.