What makes it difficult for owners is that there is no Multiple Listing Service or similar system here to show true sales figures. However, a capable agent will use three or four data points to triangulate to and verify the correct market price for a property. There is actually a real market price per m2 for each neighborhood/sub zone, then you add or subtract depending upon specific amenities of the property.
I would be happy to help you Tex except I work primarily Recoleta, Bario Norte, the Palermos (all), Las Cañitas, and Belgrano. I don't know Caballito.
Some real estate offices offer appraisals without charge but you have to be careful with those. A few agents will make a low appraisal just to make a quick sale. It happened to the elderly woman whose apartment (in horrible condition) just below mine was listed and sold for an insanely cheap price in early 2007 It needed to be completely remodeled but I think it would have sold for at least 20% more as is within three months. The same agent (his office is in the next building) appraised my place in March of 2009 for about 20% less than "what" (aka "the amount") I sold it for a few months later.
Some truth there. But in "other" parts of the world there are reference points like MLS and Zillow which will get you pretty close to the bone. Here there is Blah, Blah, Blah and more Blah.