To me, "fancy" is about not allowing money-saving measures to have an effect on the taste and quality of the food. The menu is honest. The prawns are actually prawns and not shrimp (They ARE different!). Maine Lobster is actually from Maine and not some cheaper kind. Kobe beef is actually from Japan and not "American style Kobe".
Fancy restaurants create food with flavor, rather than trying to appeal to people with weak palates that can't handle a little garlic and rosemary.
All of this pushes prices up, but fancy restaurants aren't trying to compete on price.
The difference between prawns and shrimp only exists in some regions. The terms have no concrete scientific meaning, they're both vernacular and depending on where you are both or either term may be used for the same critter. I presume you mean that prawns are the bigger ones, but I've bought local caught fresh "shrimp" on South Carolina coast that were nearly as large as lobsters. There's really no consistent widely respected rule for which is which.
From a purely scientific naming, there actually is a difference between shrimp and prawns. They're different sub-orders. Prawns are Dendrobranchiata, while shrimp are Pleocyemata. I'm not sure it makes much culinary difference to me though.
Fancy restaurants create food with flavor, rather than trying to appeal to people with weak palates that can't handle a little garlic and rosemary.
All of this pushes prices up, but fancy restaurants aren't trying to compete on price.