The average on 4d6 drop lowest is about 12.2; so the average total score for a character generated this way is 73.2. The free base 8 ability score in the point-buy system gives a total of 48 points, and scores up to 14 are bought at one point per point, so a 25 point buy matches up with the average of the 4d6 method. Which is why it's the default ... Now, if you buy stats up above 14, when the costs start to increase, you lower your average score; but on the other hand, you could end up with a 3 or 4 score rolling dice, and you don't have any control over your score distribution, so you could say it balances out.
Here's some useful information I worked out regarding the point-buy levels. Looking at the distribution of average total scores using the 3d6 method (which the worldwide average would be based on, right?), here are the percentiles in the overall population that your point-buy totals put your PC's in:
15 points - 50 (exactly average)
21 points - 75
25 points - 90
28 points - 95
32 points - 99
So, using the standard 25 point buy, your PC's will have better total attribute scores than 90% of the population.
FTR, I'm a 28-point fan myself. Actually, I prefer rolling, using the organic method ... flattens out the bottom of the curve without excessively raising the top end, and has some characters with unusual placement of ability scores (eg. the fighter with 14 wisdom).