How does that follow? It is not just about the cost, but the amount of utility for the NSA. There are plenty of free services that are not on the PRISM list and I am sure even Apple employees would freely admit their pre-iCloud user numbers were disappointing. They would not have bothered to rebrand the service in the first place if they had a significant userbase.
Looking at the PRISM company list, we are talking data service companies with users in the tens of millions (minus the oddball Paltalk). Apple just wasn't in that group until recently.