I'm definitely the exception rather than the rule. I bought everything because I knew I'd need them down the road (exempli gratia, I need white tie to perform vocal recitals as part of a class I'm taking at university; all I need to get now are a dress sporran and white vest and bow tie), and the prom at my high school is always über formal.

Seeing what you have, and based on your description, it seems that you just need a black sporran (I believe SWK has some), a shirt and cufflinks and studs (I got mine for less than $20 from uniformalwearhouse.com), and either a vest or black belt (it's really up to you which you want).

Should be quite affordable, and will look quite elegant.