The THRAWN TRILOGY by Timothy Zahn has now sold just under 20 million copies. Or, to put it another way, it's sold more than the DUNE novels and the HITCH-HIKER'S GUIDE TO THE TRILOGY series and a smidgen less than the FOUNDATION books. It's one of the biggest-selling science fiction series of all time solely on its own.

Expanded Universe material - novels, video games and comics - have made Lucasfilm approximately $6 billion since 1977. That's more than the theatrical take of the six movies themselves (rather less once DVD, VHS and Blu-Ray sales are taken into account).

Obviously all bets were off once Disney took over, but the EU has been quite enormously popular and kept Lucasfilm ticking over in the long years between films. It isn't really obscure at all.