Not that high. Spotify uses some pretty tight compression (not good, just tight); most users get 96-128kbit/s AAC, premium can go a bit higher if opted in. That works out to about 16KB/s or 58MB/hour, assuming nothing's cached.

Bandwidth pricing very much goes down with scale, not up. But even the non-committed AWS pricing at Spotify's scale is 2 to 3 cents/GB. You end up paying way less than that with any kind of commitment and AWS isn't the cheapest around to begin with.