Thought I might share my experiences in this arena, though they're non-traditional. I'm published in the romance genre through 2 different small presses (I write contemporary/sensual, which isn't one of the big-selling romance subgenres, but anyway...). My next novel comes out in May, and late last year I started thinking about getting the rights back to my earliest novel which has never sold well (variety of reasons) and re-release it myself as part of KDP on Amazon.
My primary reason for doing this was early 2013 promotion and to try and drive sales for my May release. I got my rights back in January, commissioned a new cover, had it formatted for the Kindle, and released it on Feb.12 of this year. Cost to me for that was approximately $200 ($100 for a new cover and $90 for the formatting - I had a .mobi file and a PDF done as well). I should note that I added 3 scenes to the story (it's approx. 60K words) and a minor character who's going to be the hero in my May release (again, trying to drum up interest in that book). Otherwise, everything remained the same. This means that the editing had already been done (I know this is a concern and a cost for self-pubbers).
I've priced it at $.99 through February to draw sales and then will bump it up (probably to $2.99) in March. I have 2 different blog tours planned for March and April and have booked cover ads on romance reader websites. I plan on using the Amazon 5 free day promotion at the beginning of May, the week before my new release comes out.
I blogged more here about sales so far and the promo efforts that seem to be connected to them. Overall, for me it hasn't been a painful process, but I think that's because I was already familiar with publishing. I admire everyone who's doing it with their first baby (or subsequent babies but I would caution, from reading a lot of these threads, that there's a lot to take into consideration, some of which I talked about in my blog.
I'm happy to chat more about my experiences as the weeks go on, and once my May book comes out. Any questions, fire away