Cloud Key chose instant type cost less to cast rules magic

[Puck]

I cast Cloud Key choosing "instant"

I cast Shrapnel Blast sacrificing cloud key itself

do I have to pay 1R or R for the sharpnel Blast?

[Arix]

The mana cost is locked in at R before you pay any costs.

[Puck]

if other player wanted to counter the shrapnel blast with Spell Blast in this situation, would he have to pay 1U or 2U?

[Arix]

2U. No matter how much you pay for Shrapnel Blast, its converted mana cost is always 2.