We have two end points for rates.
A getRate and getMidMarketRate
The getRate returns the rate with our margin included. You can display that rate to your users, or you can mark that rate up and make the spread difference.
The getMidMarketRate returns to you the Mid Market rate which is also referred to as the "inter-bank" rate. This is the currency rate with no markup.
You can use the getMidMarketRate to calculate the spread for your users if you want to.