Bernina accessories tend to be the most expensive. I would say Viking falls next in line. I have found that Babylock, Janome and Pfaff are all about the same price range for accessories.
You can't go wrong with Janome IMHO. If cost is much of an issue, don't disregard the Janome 6500 which has the same throat space as the 6600 but does not have the built in dual feed (which I personally dislike immensely!). The main thing is to give yourself some time to test drive each machine and see what fits you best.