Minor point really, but, assuming you listen to music 8 hours every day and never repeat a song, it will take you 20 years to listen to one million songs. So really, you need to take into account how many songs the typical user would actually own, how many they already have in their collection, and what percentage of the remaining to be purchased tracks they would aquire as downloads from iTunes, Amazon, etc., as opposed to buying the CD which many people still do, before you could actually compare relative costs. And even then, you are comparing the cost for use of a service against the cost of purchasing something which you then own. So for all those CD purschases, don't forget to subtract resale value from the total cost.
In other words, comparing costs is kind of pointless. If you like the service, don't mind the artists getting esentially nothing for their work, and the price is reasonable to you, then go ahead and sign up for Spotify. Of course, if these type of services end up killing off the LP, CD and non-DRM download distribution channels, well it probably won't be good for the consumer or the artists.