the cannon is worth less because it has capture conditions that the rook doesn't have. Of course, it is much better than a chariot in some cases! I would say a soldier having crossed the river is worth more than an advisor or elephant because it isn't constrained into path movement.
Also, NO WAY a horse is worth the same as a cannon. You can take a horse with a cannon straight off from the starting position, but it is a very bad move to do so.
(gem) Hvis du vil hilse på nogen, på deres eget sprog så prøv vores "spiller ordbog" under "mere om sprog" linket under flagene. (pauloaguia) (vis alle tips)