We stayed 2 nights at casa tranquilo and really liked the place. We booked a double bedroom with private shower for $12/night/person. It was very clean and nicely furnished. The communal kitchen is also very practical and has a nice view (with a balcony all around). Breakfast food is included, but you might want to add own food as it is very basic (toast, milk, eggs and fruits).
I would definitely recommend to stay there if you want a quiet, nice, central and affordable place in Monteverde.