I expect the sales will hold, at least enough, through S5 or so. I'd like to think that for every fan who stops buying them because it's becoming too expensive, there's another who didn't buy S1-2 because he was waiting for S3-5. Sales may drop after that, but it probably wouldn't matter.
Sales probably will drop on DS9 though, if they decide to pick it up. But maybe, in an optimistic scenario, CBS will figure they're already committed and push on ahead. But then decide to cheap out on VOY by simply upscaling the effects shots.
This is all evil speculation of course.