According to the Tribune, the price was the result of music licensing fees. Paul Feig and Judd Apatow had long ago filled us in on that when the efforts to produce a DVD set of this series were born. But looking at that article, it's quite a pervasive issue. It notes that the recent DVD releases of the Warner Brothers' network teenybopper shows ("Dawson's Creek" and the like) had their original pop music substituted to cut that cost.
While surprising, given current trends with music and television, it's not unheard of. In recent years, the studios have negotiated to keep that music in syndicated reruns, and they will probably do better about planning for DVD releases in the future. But there was a time not long ago when they didn't even do that much. I recall an episode of "Magnum, P.I.", in its original CBS airing, that used Genesis' "The Brazilian" through much of the episode's rising action. When I saw that episode again in cable syndication, the song was removed and replaced with composed score. It was years before I understood why someone had utterly ruined the emotional pace of that show.