But have you ever considered advertising on public transit? It's actually cheaper than you might think. I'm from Toronto and recently came across this article about advertising on our subway and tram system called the TTC: http://www.blogto.com/city/2010/10/ever_wondered_what_it_cos...
It might be worthwhile to check out your local subway system, if that form of advertising would hit your target market.
Maybe I'm misreading this, but promoted tweets look ripe for abuse by people who think social media is just ``another source of free traffic.'' (I've stolen that from the blurb of this article: http://blog.kissmetrics.com/social-media-abuse/.) The truth is, for me, that I automatically ignore content that is an obvious form of advertising.
To summarize, Perhaps you'll get more ROI with an active (and free) twitter account than with one-shot ads that cost a lot.
As a final question: What is the lifespan of a promoted tweet? Are they less transient than normal tweets?