You can refer to the saxophone museum tab in the upper left corner of this page.

There is a serial number sequence there that will tell you what year it was manufactured.

 It is a tru tone. From 1926

As far as engraving goes, there was a lot of custom engraving going on during the hey day of these great horns. Yours looks gold plated. The engraving may have been done by one of the great engravers Conn had on staff. They also did a lot of their own after hour work for special friends and for extra money. Selmer, Martin, and probably Buescher, paid these artists to do work for them away from Conn.

Yes it is a one of a kind piece of art, but it is still just a 1920s saxophone. In general, without the custom engraving, they are very common. 750 for a gold plate. Add maybe 2500 for the extra engraving for someone that just has to have it.

Good Luck