My guess is BASIC is free:
15% PHB (48 pages) is the BASIC PHB
15% DMG (48 pages) is the BASIC DMG
15% MM (48 pages) is the BASIC MM
all 3 as free downloads. I think this because the playtest document is pretty close to this already and quite a number of people have it. If you remove many of the class options and spells, and some of the magic items, how to play , multiclass, etc. from the playtest you'd be pretty close to 48 pages x 3 = 144 (although they each could be more or less than 48 pages).
Starter Set is a BASIC adventure with dice (thus DM only)
The actual PHB is the STANDARD game
The actual DMG is the STANDARD game plus ADVANCED variants (sidebars with how to dial)
The actual MM is the STANDARD game plus ADVANCED variants (sidebars with how to dial) - given some of the info on the MM, I would not be surprised to find simple Orcs, Ogres, Carrio Crawlers, Zombies, etc ONLY in the free PDF with Orc Shamans, Orc Chiefs, Giant Zombies, etc in the MM
Real ADVANCED rules are likely to be splatbooks, where the sidebars are expanded upon. ie there might be a tactical (4e style) sidebar in the DMG but a whole book will likely come out down the road.
my speculative 2cp