Tareq_Nahhas can you tell us if you lost $700 per passenger or $700 in total for both passengers ?
From what I can see, trying a DOH-MIA-DOH booking with DOH-MIA in Economy Saver and MIA-DOH in Economy Flexible, the cancellation fee (coming from the most restrictive rule which is from the DOH-MIA Eco saver fare) is QAR380=$100 so, if when you booked your ticket the rules were the same, here's how it should have worked for your refund (per passenger) :
.Your original ticket price (DOH-MIA-DOH): $1800
.Ticket repriced to one-way (as you cancelled the MIA-DOH return leg*): $950
.Cancellation fee: $100
Refund = $1800-$950-100 = $740
So your total loss (per passenger) should be the cancellation fee ($100) + the difference between a one-way fare and your outbound fare ($950-$700) = $350 per person
But if you consider that if you knew before buying your ticket that you would only need a DOH-MIA one-way, you would have payed it $950. So not knowing earlier that you only needed a one-way made you lost only $100 (the cancellation fee) per passenger.

*all airlines selling return fares, reprice your ticket to a one-way fare when you cancel the inbound flight, otherwise it would be too easy to game the system, buying a return ticket with a fully refundable fare for the inbound and asking for a full refund of the inbound.

If you lost $700 in total (for 2 passengers) that's the right amount of money, even if I still agree that QR website, as unfortunately many other airlines websites (not even taking in account those low cost airlines websites trying to add you as many as extra stuff like seat and assurance to inflate your ticket price with a very difficult to find button to remove these extras) is misleading when you buy a mixed fare tickets, by not telling you the most restrictive rules will apply. You still may log a complaint, suggesting they should improve their services by telling passengers what to expect when they buy mixed fare tickets, on their feedback/tell us webpage, even if unfortunately I doubt they would really care about it...