The parts that cost Sony money are the monthly games, PS+ discounts on PS Store and cloud saves. Monthly games seem to be given in a one-time payment deal with the publisher of the game so the amount of subscribers has an effect on the cost but does not directly correlate - many subscribers do not redeem the monthly games. The PS+ discount is more of a mystery as it's not a case of Plus getting more, it's a case of non-Plus getting less.
The monthly games are the most significant expense of the thing, which is why I think if they keep creating negative social media reactions they might just get rid of them. Sony already reduced the amount of games even for PS4 users because you lost the cross-buy games.
It doesn't take away the fact that it takes much more in F2P microtransactions to make up for the loss of a $60/year sub. Someone subbing for online play (which is almost all Plus subs now) is less likely to redeem monthly games which is always considered when making the monthly game deals.
