We're in the minority here. We like to dress up for formal nights -- probably because we don't dress up much while we're at home plus it adds to the excitement of the vacation. My youngest ds really gets a kick out of wearing his suit!! My older two ds don't particularly care to dress up but they do it to please me. This is a great opportunity to have a family portrait taken.
The best suggestion I can give those who want their kids to dress up and not spend a fortune on new clothes (especially if this is going to be a one-time thing) is go to your local thrift shop, Amvet, GoodWill store, etc. You can't believe how many kids suits and dresses are for sale that look like they've only been worn once or twice! Many clothes still have the original tags on them. I bought a perfectly good sport coat for my then 13yo ds for only 95 cents!! There are a lot of bargains out there. HTH