As posted above I’v traked a couple adventures with psions in them that could have benefited from the feat and tracked the times where they could have reasonably used it. This gives us an idea of the feat power if only on the lower side, because those characters were not thought around it (surely a Seer or Telepath in an urban adventure with lots of investigation would be on the upper side)
The first one is Bite of the werewolf. This adventure had a Telepath 4 (Boog) that turned 5 while in the adventure and a psiwarrior 4 (Julianna), both of them that could have qualified for the feat at the moment they started the adventure.
1st day:
Combat against giant wasps: Julianna manifests Offensive prescience, but it’s unlikely she’d have used the feat. Since the wasps were Vermin, Boog is limited to using a crossbow.
Later they rest and Julianna uses Body adjustment. She could have used the feat without problem, saving one point.
Net gain: Julianna one, Boog zero.
2nd day:
Boog manifests Mindlink (no meaningful time constraints). Using Power build optimally (manifesting it twice instead of only once, getting focus in between, etc) he could have saved 2 power points.
Net gain: Boog 2 power points.
3rd day:
A fight with ethereal monsters (thought eaters) break out. As the party is caught by surprise, it’s highly unlikely any of the two psionic characters would have used the feat; they used (or try to use) several powers though, and Boog was completely drained of power points.
Net gain: zero.
5th day:
Again the party is taken by surprise by a pack of two winter wolves. They have little time to react, and being either manifesting frantically or attacking every round (taking good advantage of Psionic weapon) it’s highly unlikely they’d have used the feat, even when they spent a lot of power points on powers and Elan abilities.
Net gain: zero.
6th day:
The party is warned by an arrow shot by an invisible enemy. Julianna manifests Thicken skin spending 4 power points, and she would have been able to use the feat to save 1 pp.
Boog uses Mindlink again. Though there was a hurry to act, it wasn’t as pressing as combat and it’d been possible to use the feat to save 2 power points.
Then the final fight, against a weretiger, happens. By the time they realized their enemy was a werewolf was a little late for buffs and such, so no Power build use. The weretiger’s name was a clue, so being generous I’ll concede Julianna 2 power points saved on buffs (that she didn’t actually manifested)
Net gain: Boog 2 power points, Julianna 3.
Based on this adventure the usefulness of Power build is comparable to Psionic talent for 3-5th level characters. It’s likely to increase for higher levels, adventures with more encounters or encounters with ample preparation time, or with more opportunities for non-combat powers.
I'll post more adventures later.