Guys I just red the spell discription and It doesn't mention material costs for enchantments greater than +5 in abilities and +10 in skills. Maybe that's the cap used? Also the best way to use Extract gift is to apply it to someone else preferably an NPC you hate and have the demon control him all the while you control the demon (preferably by making it undead)... Which reminds me: does a demon still coun as a demon if it becomes undead?
It give the formula to figure out the price, then it does the pricing for you up to CL20. There is no cap for the spell itself.
As far as the undead thing, I would say no. However, it is up to the DM. Since it is up for interpretation, I do not include it for the same reason that the Aspect of Lolth is not in my chart.
Fiendform et al allow one to become demons.
But only a specific list of crappy demons. Since Polymorph changes type, Draconic Polymorph is love.
Also, I have been refining my table, making many more at each binding level, and many more things to make it a better handbook.