Farmpony, I started dressage lessons 6 months back with my qh (mostly, although I took my paint there too), and it's unbelievable how much better she moves (including those transitions). I also took lessons with eventing trainer and jumping trainer before and those were not nearly as helpful. So I'd say go for it!
Now I don't know about your area, but in mine private 45 mins with dressage trainer ranges in $50 - 80. I'd look for someone who won USDF competitions, have years and years of riding and training, and who's students show successfully (at least on local shows). Also the trainer should be supportive and optimistic to work with ANY horse (as some don't care about "non-fancy" breeds