I dunno if Powerful Build grants reach or access to bigger weapons, but....
If not, then approximately 3 feats in value; possibly 4 but that's likely stretching it. Sorta similar to Imp. Grapple, Imp. Trip, and Imp. Bull Rush together, and covering the other stuff too (increased carrying capacity and whatnot).
If it grants access to larger weapons as well, then definitely 4 or 5 feats in value, from the extra +1 or +2 to average damage that it would yield, and the greater minimum and/or maximum damage it would allow. I'd probably say 5 feats, or 4-1/2.
If reach were also thrown in, then that'd bump it up another 1-3 feats in value. Probably worth 2 feats, so a total of 6-1/2 or 7 feats.
Of course, you also have to consider how well Powerful Build meshes with the other qualities of a race. If the race is meek and has nothing else of use in combat, then PB isn't quite as significant since the race won't likely be getting maximum benefit out of it. But a strong race, for example, would have better synergy with it and get the most out of PB, warranting the full value of PB being considered in determining the race's LA/ECL.
Normally, a larger creature would suffer AC/attack/Hide penalties that would balance out 3-5 feats worth of the size benefits (depending on just how big the critter was), but Powerful Build ignores the usual balancing factor for no reason at all (it really doesn't make any sense to me why a critter would be considered one size larger for all beneficial purposes but without any of the drawbacks).
*sticks his tongue out at Goliaths in annoyed derision*