Apple Music and Google Play Music are each approximately half the size of Spotify (by subscriber). Assuming that the revenues for each service are similar (on a subscriber basis) and overhead would be similar if they were spun out, then you are just valuing them based on the number of current subscribers. Given the low friction in switching services, subscribers are around the same value for each service. My guess based on this back of the napkin math then, is that $15B for would be the comp for each of Apple Music and Google Play Music, though you would need to consider any debt held by the companies (standing for Spotify, or as part of the spin-out by the other two).

That's just how I think the comp would work, I don't think any of those companies are worth those values, including Spotify.