If you are willing to stretch your definition of a sandwich to a coxinha (a manioc ball stuffed with chicken and then deep fried), then Oliveira Deli and Market in Brighton off Oak Square has softball size ones for around $5. Get two of those and a cold can of Guaraná Antártida for about $12 total.

They also apparently do some good sandwiches, but I've never gotten anything except the coxinhas due to their deliciousness and being a bit set in my ways.