Tickets are going on sale in the next couple of weeks. They will all be sold out in minutes, mostly to Cariocas (Rio locals) and agencies. As a tourist you are very unlikely to buy at these original face prices. Expect to buy from an agency with a mark-up of at least three times the face value. Its already possible to reserve tickets for the sambodromo through online agencies.
Original face values:( R$ = Brazilian Reals);Frisas (for 6 people) from R$3000 to R$6,500 per night, Arquibancadas (stands or bleachers)- from R$110 to R$300 per night per seat.