sjr - now is the time to hit the end of the summer season sales at your local department store! You can find some great deals on summerwear.
Formalwear - Platazzo pants(full-dressy pants) are very much in style right now, add a dressy, silky or sparkly top, dress shoes, some nice jewerly and you are all set. On a 7 day cruise you will have two formal nights. A nice dressy pantsuit will also be just fine. A dressy cocktail dress always works. Another option would be to rent your formal wear. Ladies can rent long black skirts with a dressy silk blouse.
For your man - if he doesn't own a dark suit, try shopping your local Goodwill, Salvation Army or consignment store for amazing deals on nearly new suits. He could also consider renting a tux. You will get rental information in your cruise documents or you can do a search under cruise formal wear. All you have to do is supply your size and measurements and your formal wear is delivered to your stateroom.
Think of formal wear as something you would wear to an Evening church wedding with a reception at the local country club or fine hotel.