You might want to drill a narrow but deep hole in the affected area to
see how deep it goes, and to check for structural integrity. If it is
dry rot, drying it out will not help.
It sounds like you need to buy some time. Believe me, I know about
money problems, and a quick cheap fix is better than none at all. Dry
it out, caulk and waterproof as best you can, and apply the Termin8t
(Hardware store, under $10/quart). Some well placed mothballs placed at
or very near the rot will slow down, but by no means stop, the
deterioration. Save your nickels for replacement the proper way later
on.-Jitney.