If you listen to many artists complain about Spotify, you'd think that they're making almost no money at all from the service. That's probably true for artists that don't get many plays per month (and when it comes to streaming, a million is not that many), but for others with a real hit on their hands it can be substantial.
Spotify has a nice page that explains how it pays royalties, and on it there's a chart that shows the payout of 5 different "hits." Of course, the definition of a hit is different for different types of acts. For instance, a hit for an indie band is gauged far differently than a hit a mainstream artist.