Depending on how long of a cruise you take, you will have either one or two formal nights. For the first formal night, I always wear a longer gown because that's when we take most of our pictures. For the second formal night, I often wear a cocktail dress.
Here's some outfit ideas for a formal night with a long gown. Some of these are over $50. Long, formal dresses are hard to find for very cheap and after your accessorize, it can get kind of expensive. So, I went up to $100 on some of these outfits.
Outfit #1, $50
Dress -- Forever 21, $35
Shoes -- 6pm, $15
Outfit #2, $74
Dress -- David's Bridal, $50
Shoes -- Target, $18
Earrings -- Forever 21, $6
Outfit #3, $98