There's also the point that he wasn't dead or completely new as well. He was basically starving to death, and reversing that trend isn't the same as being the direct source of his apparent strength. If you rescued a body builder from dying of deprivation you couldn't reasonably claim that the food and water you gave them made them stronger even if they needed it to start exerting themselves again.Aaand here's where our understanding of the matter differs.
If rezzing a God was as easy as that, we really would've seen Meraxes rezzed long before we came to Volantis, nothing to say about other more minor God-stuff.
Summoning and capturing Quazits/Imps in such quantities is really, really not a problem even with minor spells, let alone for mid-to-high level casters.
Just gotta ignore some moral boundaries like we do, and torture out/Brainspider the True Names of random low-cr Outsiders, which we don't do.
I can see something like our 2.000 HD-meal that was spent on raising Dawn Age Heart Tree waking up the God a little.
I see a reasonable amount for an actual decent boost for God's existence in 3-4k range at the lowest.
The way I see it, what got Yss so pumped was that:
a) We got a very hard-to-ignore-and-disbelieve-at Avatar for him at the soon-to-be cultural centre of the World.
b) We had people doing acts of worship to him for years, doing sacrifices from minor to pretty major in the background, with each minor Divination given also strengthening him in return as a "fair deal" completed.
c) And those being the main things really, we also threw some really fat pieces down his gullet every now and again.
The cost of waking a god back up is probably a function of the context factors you brought up and how much "muscle mass" is left to reinvigorate after ages of disuse and cannibalization. Yss just happened to be stupendously fat snake before he went on his surprise diet.