I think the ideas here are really great, too.
So a question remains unanswered; EXACTLY how much should the XP penalty be, if the caster is unwilling to do a service for planar cohort? 200 XP/ Cohort HD? Or is that HD of defeated monsters? Could this be paid off in Magic items/jewelry/coin? At what exchange rate?
Diplomacy check would be 10+CR of planar cohort, and reduce the XP penalty by 25% (to 150 XP per HD)
This opens up the oportunity for the
Extraplanar Negotiator feat;
Requirements: Knowledge (the Planes) 10 ranks, Diplomacy 5 ranks.
Benefit: granting +4 bonus to Charisma checks when dealing with Extraplanar creatures, and reducing the XP penalty for not returning a service to a planar cohort by 50% (100XP per HD) on a successful Diplomacy check.
I like the owing a service; It would be pretty cool, the summoner could end up getting summoned by his planar cohort at an importune moment...
I think I'd rather have it as a psionic feat, but as a power would be great as well. +2 would place it as a 2nd level power (as ecto-puppet). Suddenly there isn't much flexibility for the poor metacreative psion wishing to specialize in Astral Constructs... Perhaps a feat would be better after all. Why not have both?
The Astral constructs/Elemental template would need some consideration... Isn't the ability to fly normally one of those you purchase? The air template would grant that for free... But I didn't feel that it was a problem for arcane spell wielders...Hmm... I'll need to grab a look at my books again.
On second thoughts I think that the XP cost should be lower...around 20 XP per HD, I mean you want the summoner to actually use his cohort on occasion. Secondly, if he can "buy off" the service, with magic items and suchlike, it should be at a cost of 50gp per XP. This is in line with the cost of Item manufacture, and would be 1000gp/HD... quite expensive....