If the digital download of the game costs £25, I don't see any expansions to cost any more than that. They'll probably end up costing less than the price of the original game.
So based on that assumption, with the price of the digital download pack you could buy at least 3 expansion to the game. That probably covers the first year of the development after the official release of the game at least..
Offcourse, who knows maybe they'll be releasing expansions for years and years such as in EVE online. Then it would probably be a very good deal. But then again, if I'm still playing Elite Dangerous after the first year of the release, I don't mind chipping in another £20 every so often for major expansions to the game.