That is such a tough question... Obviously there is no
right answer but usually I will look up the piece and see what other people sell some similar frags for. Sometimes you will find a similar frag on other trading posts or frag sites. Then I look at the price others have charged/paid and ask myself if I'd buy it for that price. Since you already have the piece it should be pretty easy for you to decide if it is something you'd buy.
IMO it comes down to if you wouldn't buy it for that price why would others?