You haven't paid any more than the original GR to cast Pentad Prism, all mana you spent afterwards was on the ability of Power Sink. While you may think of the two as related because one targets the other and affects whether it ends up on the battlefield, they are two separate entities on the stack, and the game doesn't allow for creative interpretation of such connections in any way.
A ruling on Power Sink clarifies this:
10/4/2004 Does not increase the mana cost of the spell. It just requires a separate expenditure in order for it to succeed.
Additionally, here's the relevant excerpt of the casting rules that illustrate how paying costs works:
601. Casting Spells
601.2f The player determines the total cost of the spell. Usually this is just the mana cost. Some spells have additional or alternative costs. Some effects may increase or reduce the cost to pay, or may provide other alternative costs. Costs may include paying mana, tapping permanents, sacrificing permanents, discarding cards, and so on. [...]
601.2g If the total cost includes a mana payment, the player then has a chance to activate mana abilities (see rule 605, “Mana Abilities”). Mana abilities must be activated before costs are paid.
601.2h The player pays the total cost in any order. [...]
This all happens before you put the respective spell on the stack, and therefore before your opponent may attempt to counter it, and the mana paid for these actions is what the game considers mana paid casting a spell.
Additional and alternative costs would have to be included in the card's text (or another permanent's text), so in order to ever spend more than two mana on Pentad Prism, you have to resort to cost increases, as found on cards like Sphere of Resistance.
