A standard array is 13, 12, 11, 10, 9, 8, worth 15 pts. An elite array (worth a +1 CR) is 15, 14, 13, 12, 10, 8, worth 25 pts. If, for this small scale, we presume +1 CR to be equivalent to +1 LA (not a huge stretch, particularly for creatures with a low LA), then 10 pts in a point buy is roughly equal to a +1 LA.
A nice and neat way of doing it would be to give LA +0 characters 32 points, LA +1 28 pts, LA +2 22 pts, and LA +3 15 pts, then just limit the PCs to LA +3.
The acid test would be to see if it is always worth it to go for the LA. In the case of the aasimar, if you are playing a paladin, it could be, particularly if you plan on a base Charisma or Wisdom score of 14 or higher. If you plan on a base Charisma of 16 with no other score higher than 14, then you have a net gain of +2 points on your point buy (i.e. it costs 6 points to bring a 16 to 18). But the other benefits are marginal: acid, cold, and electricity resistance and daylight once/day. Many players would rather have a bonus feat and bonus skill points as a human.
The question is whether or not you think this will encourage a lot of aasimar paladins (and similar builds). If it means that all paladin players will start prefering aasimars, then it is probably not balanced. I prefer to limit such characters in my campaigns and try to find ways to encourage my players to play core races. Of course, if you run a froo froo world like FR, it might not be as much of a problem to see the sons and daughters of angels, demons, and elementals frollicking about gaily in the meadows.