It's really hard to gauge what a band actually sells anymore, between online downloading and cd burning, etc. But from what I've been told, 'Blackwater Park' sold somewhere around 18,000 to 25, 000 in the U.S. I have no idea what the international sales are like. Its quite possible that they've sold an additional 50, 000 in the rest of the world. Bands like H.I.M sell hundreds of thousands of albums in europe, but don't even break the 10, 000 mark in the States. I know the Moonspell cds havent sold over 15,000 a piece in the U.S.
Another problem with figuring out these sales figures is, bands like Opeth are sold, to a large degree, through mail-order companies and smaller "mom n pop" record stores, and none of which usually use Soundscan. Even though Soundscan is supposedly modified to try to account for alot of the unregistered sales, when it comes to the underground acts I am sure it can't be nearly accurate enough.
Also, even if a band like Opeth has sold 100,000 copies of a disc, you'd be surprised at how little revenue that actually accounts for in terms of paying the band members. Most underground bands, even the extremely popular ones, have day jobs. I do believe that some Euro musicians can collect some form of revenue from the government though, almost like a musicians welfare or something...
Which makes me think that relocating to Sweden might not be a bad idea at this rate! hehehehe