Unless you have lots of black tie weddings and galas on the calendar, the price per wear of an appropriately formal outfit can be very high.To be perfectly frank, even with the suggestions in this post it is likely that the price per wear will be much higher than you normally would choose to spend.
First off, before you go shopping at all, double check the dress code and see if it is black tie required or optional. For a black tie optional event, you can probably scrounge up a great party dress from the back of your closet. Black tie required pretty much means that you will have to don a full-length gown, because the exceptions to that rule are finnicky and not worth the effort to navigate.
Rent The Runway
There have always been options for men to rent a suit or tux for a special event, but now there is a finally an easy option for women as well. Rent the Runway lets you order a dress online (or in select cities, in store) that will be mailed to you to use for a set amount of time (several days to a week). Instead of shelling out upwards of $400 for a gown, you can rent it. They have lots of black tie options in the $80-$100 range so there is something for every taste.
If you would like to invest in a dress, instead of just renting, RueLaLa is a great option. However, this requires that you start browsing their website several months before your event. The best time to find a full-length gown is during one of their big dress sales, which usually happen once or twice a season. There are generally a few options under $100, but for a better selection, expect to spend around $150. The nice part about buying a dress is that you have something on hand for a future black tie event.
My point here is that you or one of your friends may have a floor-length gown that can be repurposed for your event. Add your personality to the dress with statement jewelry and a unique purse. Yes, it may still look like a bridesmaid dress, but if any of the other guests notice, it will probably be to note your ingenuity. Even if the dress requires alterations to make you feel confident in it, your wallet will still be coming out ahead.
When dressing for a black tie wedding, it can be easy to get caught up in the dress code and feel like you need to spend more than you would otherwise be comfortable with. But just because the wedding will be formal and lavish does not mean that you need to pour your yearly clothing budget into dressing for it. Rent a dress, borrow a friend's dress, or become a sleuth of the sales rack so that you can find an option within your comfort zone.