While the exact details haven't been revealed yet, it's believed that the service will offer a free tier with a capped number of streams and downloads per month, and a paid tier where both are unlimited.
One of the challenges is changing current users into paid subscribers, since the service has long touted itself as a free service. In fact, Soundcloud has used that as a selling point from the beginning.
The company is currently under pressure both from investors to make money, and from the major labels pay royalties, however, so it looks like this will be changing soon.
That said, Soundcloud hasn't been entirely free for some time, as it offers both Pro ($6 per month) and Pro Unlimited ($15 per month) tiers for creators that want increased storage and analytics.