Salad crops are relatively easy to grow and expensive to buy;
Courgettes are probably the most productive and not too hard to grow;
Peas are a bit more of a fiddle as you do need to pick twice a day once they get to producing and have lots of freezer room but cash wise they are a good saver.
Depends on the space you have, your talent and what your family eats though as to what will be the most productive for you.
Grow the ones you like that are easy enough and yet the most expensive in the shops when in season.