How much should this fix cost?

[67_C-30]

Being that the truck is a C/40 - C/70 it is likely that it is just water damage from the topside. Most of these trucks do not rust from the bottom up as bad as C/10-C/30 trucks due to them sitting much higher.

If were mine, and I didn't have the time to fix it correctly, I would have someone lap some sheet metal over the rusted areas and weld in patches until you can stop using it long enough to fix it correctly. Unless the rust is into the cab supports, etc it is not going to hurt anything to patch it up for a while.