Aren’t their costs going up at least 25-30% because of Meltdown/Spectre mitigation performance penalties? Would they weather another 10-20% price hike on top of that?

25-30% is pretty pessimistic. Maybe on certain services, like a Redis box. On others, like your average web service, it probably won't move the needle much.

I'd ballpark around 5%. And that's for people who weren't overprovisioned in the first place, which is most people.