Sorry did not realise you are a 58cm....not much chance of a 58cm Keirin frame for 800 bucks. They go through the roof on eBay when they are large sizes, because every Gaikokujin rider needs 550-580mm it seems.
Also Fix/SS riders are notorious cheapskates(compared to roadies), so do not stress too much about the opinions you get, when regarding price. Maybe a Cannondale is not worth that much, but a lot of frames are....