I've posted about this before, but here I combine info from multiple things I posted:
Summary: 4d6 drop lowest works out to about 18-19 points if you don't allow rerolls and instead assign points to low numbers (3-6) so that it's possible to score very low rolls. If you were to allow rerolls (of the entire rolled array), the point value would probably be higher.
Point costs are based on the modifier of the next score, plus 0 if negative, plus 1 if non-negative. If you are going from a 10 to an 11, the modifier for 11 is 0, add 1, so an 11 costs 1 point. This is step-based, so buying a 14 costs an 11, plus a 12, plus a 13, plus a 14, plus 1, resulting in 0 + 1 + 1 + 2 + 1 = 5. But we actually don't need all this: the point values for 9 down to 3 are the same as 12 up to 18, minus one, then made negative. So: 6 -> -6, 5 -> -9, 4 -> -12, 3 -> -16. The more laborious "adding up the modifiers" approach confirms this simplification.
Anyway, with these point values, it is possible to roll 4d6 drop 1 and score the resulting array, and I wrote a little Python script to do so. It was pseudorandom, admittedly, but probably good enough for this. The result was 18-19 points (18 if flooring the average, 19 if rounding to the nearest integer). And keep in mind, this includes the possibility of rolls lower than you are "allowed" to buy using points, which brings the average point cost down. So if you reroll arrays that can't be point-bought, the average score probably goes up noticeably, at least a little bit.
Now, this was probably known when the rules were written. You can roll with greater risk for greater reward, or you can select the exact values you want, and this is tempered by having fewer points to work with.
That said, I don't think there are many major differences between 15 point buy and 20 point buy, other than planning flexibility for feat requirements and such. This is certainly affected by my view that increasing your high stat even further is non-optimal because that suffers from diminishing returns and your points are stronger a little more spread out. Depends on the class and goal for the character, of course.