I visited Plaza de Mayo my first day in Argentina with the Cabildo being my first stop. As most guide books point out, it is the only surviving building from Colonial times on the Plaza and that fact alone makes a visit worthwhile. The cost is around 4 pesos. The building houses a small museum that is interesting but unfortunately had no English subtitles with the objects and being a non-Spanish speaker my return was minimal on these. A visit to the second floor gives a nice view over the Plaza but overall I only spent maybe 20 minutes here. The building is worth visiting for its historical significance although a quick visit should suffice for most folks.