I've been looking at convertible bridesmaids dresses (so that I can get the same dress for both of my bridesmaids but they could wear it different ways -- and yes, I only have 2) like these http://www.onewed.com/blog/savvy-scoop/category/bridesmaids/2010/09/10/convertible-bridesmaid-dresses-bride-chic/
but they were in the $150 range...
And I found some on Ideeli for only $69 (or even less!) Unfortunately the colors don't work for me, but maybe they will work for you.
By the way, I bought a dress from them a few weeks ago and got it very quickly -- even before the estimated ship date!