I don't have a recommendation in capital but usually it is cheaper to get a remis from where you are going. When I lived in the province, the local remises were much cheaper and only charged one way, while if you got a car from capital, it would often charge both ways. So maybe look at the area where you are going and google remiserias around there? Or ask the estancia if they can recommend a local remiseria. Another benefit of using a service from that area is that they will know how to get there; a taxi from capital might have problems finding the place (depending on the the location of course - some of them are on dirt roads etc.)