On the JD configurator it is $267, but looks like you need to drill a hole through the hood. I think the older 790 had the exhaust come out through the side panel.
Vertical Exhaust Kit LVB25829 $267.00
For applications that require a high exhaust system, over the head of the operator, a vertical exhaust kit is available.
Ideal for use around sensitive vegetation to direct exhaust up and away from plants Can be indexed 90 degrees to the front or side as desired
Code Bundle Description
LVB25829 Vertical exhaust kit (3005)