I second workandski's point regarding cost of accommodation in Rio (and in Brazil in general) over carnival. Most of us Brazilians try to flee during carnival as hotel and B&B owners really lose all sense of decency and multiply the high season rates by 3 or 4 times, and require a 5-night minimum stay. Same goes for airbnb.

It's not infrequent to see U$50+ rates for a dirty dorm bed during Rio carnival.