You see the US Dollar rate of a cryptocurrency that is much lower than you thought it would be. You have found on www.coinmarketcap.com and www.coingecko.com an expected rate. The exchange you use does not have fees. You want to convert a cryptocurrency into a different coin. But the value/rate is discrepant with other sources. Why is there an error in the rate for the exchange website you use?
Many exchanges have a spread. Some companies deceptively advertise themselves as having a small spread. This spread can be 10%! You may be able to overcome this high cost (a pricey conversion fee that the exchange hides) by using an advanced trade with an order to convert. There may be a fee, but the fee may be low enough to preserve the value of the exchange in a way that is satisfactory to you and consistent with third party sources in terms of the dollar value. If the trade you set up is set at a specific price point, you may be able to get a higher price for your cryptocurrency.
You may want to take a screenshot of the transaction when you are reviewing it (or previewing it) before you confirm it. This way you know what you should get before it is finalized. You may need to speak to an attorney if the exchange is not being fair. Keep records of your transactions before they happen.