A one way direct flight to Rio (starting at € 400) is ridiculously expensive. The more affordable options you are given if you use one of the search engines as kayak (starting at €350)are usualy with a one stop flight either via Santiago de Chile or Sao Paolo ( even via Panama City with COPA) with a duration of 10 to 12 hours with LAN,Turkish airlines,Qatar airways even Ethiopian airways. If there is no rush and there is an adventuies side of yours take the 20 hour bus ride from Retiro to Porto Alegre,Brazil and from there fly to Rio into Santos Doman airport (the equivalent to Aeropargue next to the city center) for less than Us$ 100.