Paella is a great idea. I just out sausage, shrimp, and chicken in mine, we are not keen on most shellfish. I add roasted red peppers and peas.
Costco has some really good mini small crab cakes. I assume they come frozen, a friend of mine served them at a party, and they were very good. They are abut the size of a quarter, perfect bite size.
A lot of the seafood in my market is not much more expensive than a good cut of beef. I like salmon, cod and tilapia. A large piece of salmon can make a great presentation. I put the fish in an aluminum foil lined cookie sheet or pan (best if it has at least a small side) some salt and dill on the fish, then put tons of already slightly salted tomato slices on and around the fish, a little fress squeezed lemon and back. So easy, and makes a nice presentation. Tilapia and cod I do the same minus the dill.
I make tuna croquettes that are half potato, they are fairly inexpensive. Peel and boil a large baking potato (cut into think chunks to cook eavenly) and then mash (nothing added). Saute a small amount of diced tomato and onion, little salt. Add a can of chunk white drained tuna to the pan and continue to cook for a minute, breaking apart the tuna well. Then add all that to the potato and mix well. Let mixture cool in fridge. Then form into patties and dip in italian breadcrumbs and fry to cook. My MIL likes to make them the shape of cylinders, I just do simple patties.