We stayed in a 2-star a couple of years ago on Avenida 25 de Mayo. Location was great as was the price. In retrospective, one of the 3-star on 9 de Julio might have been a better choice and unless the prices have changed drastically the 3-star will be almost in your price range.

On the other hand the following link will not make any recommendations but will certainly provide 99% of your hotel options..

La Asociación de Hoteles, Restaurantes, Confiterías y Cafés es la entidad que, con Personería Jurídica Nacional, representa desde 1905 a los hoteles, restaurantes, confiterías-pastelerías y cafés de la ciudad de Buenos Aires.

Hope I didn't wander too far......

Fred