If I found an Oxxford Crest (their only model that doesn't look frumpy on me) that fit me, in a fabric I liked, I would pay $900 in a heartbeat. Of course, I also paid much more for a MTM Oxxford Crest because I'm a difficult fit. $900 for an Oxxford suit is a very good price, although Filene's will reportedly discount them to something near $600, if you are patient and willing to risk losing the suit in the interim.
If in good shape and fairly recent (last 3 years) then $900 is a terrific deal.
And the fit was good, the cut flattering and the fabric something that I can use, then yes, I'd buy it. Though...I have let a few (three to be exact) go by at about $650 that fit that discription (on ebay).
An Oxxford at $900 is a good deal, but not so spectacular as to worry about passing up. If you had found one you really liked, it would certainly be a huge savings off retail. But Oxxfords can be found at that price fairly regularly, and, as Thinman mentioned, often for less. (A solid would be a find at $900, though.)