I reckon it will be an add-on to ps+ to be honest, We all need ps+ to play online and for the streaming service to be either a sub or pay as you go it would have to be something seperate.
I'm not sure how cheap it is going to be either as you are streaming all the time and bandwidth costs money. I'm hoping and extra 10-20 quid a year but can see it being a tenner a month.
On a side note I saw something about isp's maybe complaining and going back to capping users due to ps4 and xbone downloads, games being 50gb etc. This could affect PS Now if they do.
A comment in the IGN link made me smile