here is a street near Sete Rios Bus Station called Avenida José Malhoa. There are several hotels there, including an Ibis, the IBIS Malhoa and a Novotel. If you check booking.com or similiar sites you may find very competitive offers. I have stayed at both. These hotels are about 6 minutes walk with from Sete Rios. Make sure you have a backpack or suitcase with wheels. However, I would agree with Cubsur's comments about prices and I would not expect to get air conditionioning for the price range you are quoting..
To find Rossio Station, find Avenida Liberdade on a map. At one end is Praça Marques de Pombal and at the other is Praça de Restauradores. Rossio is a few metres to the west of Praça de Restauradores.. The map you are looking at does not show Rossio Station as it is off on a spur.
The metro station you need to get off at and which has a direct link to Rossio Station is Restuaradores on the Linha Azul (Blue Line.) This runs straight down from Sete Rios. See the map here: http://bit.ly/c8nFoH+
