Well 68 for a 69 could be considered even , depending on condition, but I would say you did well with the Monza for a 70 Z. The 70 Z/28 have shown to have a strong market as of late.
My friend traded his 67 Z/28, for a 68 road runner, (spring 1970)One of his biggest regrets. Must be other guys out there that traded cars back in the day