If you're confused about what Afterbirth+ is, its essentially an upcoming piece of DLC for The Binding of Isaac: Afterbirth that will bring with it new items & enemies, a bestiary, a couple of features they are being very hush-hush about and most importantly mod support along with tools that will allow you to fiddle with the game's code in order to create whatever monstrosities lay in your imagination.
Details about the DLC itself were rather scarce when it was first announced but in a recent blog post Edmund McMillen, who is now apparently evil, shared a few tidbits of information on Afterbirth+ including the fact that it is going to be a paid piece of DLC rather than a free update as many expected.
The reason its going to be a paid piece of content is because they are treating it as an actual expansion, smaller than Afterbirth itself, but still rather large. Or as Edmund himself put it:
"Because it’s another expansion, just like Afterbirth and will complete the Rebirth ********* and ********* ***. As far as gameplay, we’re adding (and this is still Tyrone, but ** might ****** the following) a bunch of new ***** items, over ** **** enemies, rad bosses, redesigned ***** mod ******* and a few ***** surprises. That’s a big part of the D**."
No actual price point was released but given Edmund's response to the question I can't imagine this being a very expensive DLC, especially considering that the content-rich Afterbirth still launched at a rather affordable €11 / $11.
How much will Afterbirth+ Cost?
"Just enough to upset a vocal minority and buy myself a 3rd yacht!"
The currently confirmed release platforms are the PC, Linux and Mac whereas the Xbox One and PS4 versions will most likely be happening although whether mod support will be available for them is still up in the air. Speaking of consoles, the Afterbirth expansion will be releasing in the next couple of months but as of right now I have no concrete date to tell you.
As for the release date of Afterbirth+ the best you're going to get right now is sometime in 2016 but I wouldn't expect it to come out soon given that the team is still rather busy with the Afterbirth console port. Once more information comes up I'll make sure to update you.