I forgot to add, many people become frustrated with Band-in-a-Box because they purchase the Pro or Mega PAK and are frustrated by the lack of content (RealTracks, MIDI SuperTracks, RealDrums and styles). Purchasing at least the UltraPlus PAK is the most cost effective method to have access to all the content released to date except unreleased or bonus content.
Once you have all available content the most cost effective way to remain up-to-date is to purchase the PlusPAK. The Plus PAK only includes whatever content is new for this release.