Does {Heartless Summoning} make {Primordial Hydra} come out with 2 more +1+1 counters if I pay {G}{G}?
In short, yes. The process is a little more than that though. This is the process for casting a spell with X in the cost:
1: Choose a value for X.
2: Determine the mana cost or activation cost for the spell or ability, substituting the chosen value for X.
3: Apply any additional costs or cost reductions.
4: Determine the total cost for the spell or ability.
5. Cast your spell for the total cost determined in step 4.
RESOLVED
Yes. Cost increases are applied before reductions.