I only support point buy when you complete the scale: that is, you can buy up AND down, using the same scaling increase of cost, all the way to 3 and 18, just as you could theoretically get for dice.
This means, from base 8, buying down gets you one point back per rank down to 4, and two back for the rank from 4 to 3, while above 15 it costs three points for 16 and 17, and four points for 18
I'm not a fan of getting additional points (on a 1-to-1 basis) for taking scores <8. This just incentivizes dump stats even more than currently, and 5e already has a big problem with certain stats being dump-able. At the very best, going from 8->7 should get you a fraction of a point. You'd need to go from 8->6 or 5 to get one point.
I do agree that point buy should allow purchasing scores up to 16 at least, maybe even 17 or 18. Possibly we could get 3 extra points to spend with this: either a free upgrade to a 16 or getting a couple additional stats to 11-14. Particular, this would very much help with MAD classes and encourage more feat-taking at ASI levels.
