Roman said:
What single ability would be worth +3 or +4 LA? How much of a bonus to a single ability score would you need to give to grant +3 or +4 LA to a race?
Well, half-dragon is a +3 LA. Interestingly enough, the appendix for Savage Species (p. 213) suggests that without the breath weapon and immunity you get from the type, it's only +2*. Since half-dragons also get natural weapons and armor and a change of type, it's arguable that the attribute bonuses alone are worth +1.
Looking at Chapter 2 of Savage Species, you'll see that the worth of ability scores depends mostly on how balanced they are and what they're in. A creature with a +20 Str will probably have a +2 or maybe even a +3 LA for that, but one with a +20 Cha would probably have a +1 or +2 at the outside.
I suspect that there'll be a point of diminishing returns; yes, an uber-Strength or other stat is nice, but it's not going to be worth as much as, say, having four more levels on you.
However, note that the elder stone giant's bonus attribute array of +16 Str, +4 Dex, +8 Con, and +4 Cha is worth a +2. Of course, it's also attached to a Large creature; I suspect that the same array in a Medium creature might be a +3 (since the Large creature really should be fairly strong and tough).
Basically, it's the DM's call. I'd suggest perusing Savage Species, especially Chapter 2, before you come up with anything.
Brad
* - The DM for our epic game ruled that, and so I gleefully took advantage of that. But that was meant to be a cheese-fest, anyway.