If you're passing through Europe and plan to go directly on to Marrakech, keep in mind that the low fare carriers more often than not use secondary airports which will add to your travel time if go straight on. For instance, if you fly into London's Heathrow airport a low fare airline will likely use either Luton or Stansted. I believe British Airways has flights from Heathrow so check on the variables and you may find that with time and costs involved to change airports a bit more expensive fare will be well worth it.
If the amount you'll save by flying into Casablanca is significant there's a train station in the basement of the airport with trains to the main Casa Voyageur Station. From there you can change to trains south to Marrakech or north to all cities north. The places you can get to by train are limited to the main cities but those are likely the places you'll be going anyway. So don't worry about arriving in Casablanca and continuing right on to someplace else. Consider it an opportunity to ride a Moroccan train which I love to do.