Bought two formal dresses in high school (10 years ago... Jebus...) One for my prom, on for my BF's prom (which was the following year at the same school).
1. Was a bridesmaid dress in a lovely light green, strapless, drop waist, knee-length -- pretty tasteful, considering the styles then. I wore it to formals in college (sorority girl here). About $150. I loved that dress, and still would if I could fit in it (it's long since been relegated to a formal dress charity for low-income girls in my hometown).
2. A stupid ugly gold floor length dress that I thought made me look old Hollywood and, in retrospect, made me look dumb. $80.
I did my hair and make-up both times, used shoes I had around (my "nice church shoes"), and had dinner either at someone's house with friends (the first year it was my house, and my mom was the BOMB about making the food excellent and still made the dinner feel "upscale" - we got to use her old china!)
All other dresses were borrowed from friends. FTW. Must be indicative of my value in formal clothes; my wedding dress was not white, still in my closet because I teach in it, and cost all of $150, including tailoring.
A good friend of mine at that time (who had done pageants) spent a fortune on her dresses, shoes, hair, makeup, jewelry, etc. Whatever--at the time, I just thought "that's who she is and what she values" and she never made me feel bad for not doing the same thing. Probably my earlier instantiation of "Not my circus, not my monkeys."