The way we resolved it, is that at the next turn, my opponent payed
1 for the second Rift Bolt.
I'm not sure if this is correct, because the rule text for supspend says that when you remove the last suspend counter you cast the spell without paying its mana cost. So, to me, it sounds like even if the cost was increased, one still does not need to pay for it.
What's the correct thing to do?
If actually one needs to pay for the second suspended spell, what would happen if he can't?