EVERYTHING IS SUBJECT TO CHANGE BEFORE RELEASE.
Primaldemic will have a fully developed story mode that can be played solo or co-op available on its proper non-early-access v1.0.0 launch. This storyline can cost anywhere between USD$3.99 to USD$14.99, depending on the total length and content. This is just a broad range due to the Story being nowhere near complete.
F2P will have the standard core gameplay experience of Primaldemic, being that there'll be a vast hub world to explore with its own (albeit not as in-depth) storylines when you're not in the main gameplay experience of crawling though procedurally-generated and hand-crafted dungeons.
Primaldemic may have some paid bonus levels you can purchase and play, but it's undecided. Most of the time, new content will just be free updates.
Playable characters will be unlocked as you progress through the Main Story. (Or, for Free-to-Play players, through challenges or non-premium currency purchases.)
(POSSIBLY; UNDECIDED) All playable characters can be instantly unlocked for non-story play through a One-Time Purchase Character Pack that will include any and all future characters after the pack releases.