Probably 1/2 of retail or more, depending on the buyer and condition of said unit.
More than likely the carbs are gummed up from sitting with gas left in tank for a period longer than 6 months. The shelf life of gasoline is 6 months, so drain the tank and carb when next use is undetermined, this goes for any gasoline application.
The repair cost might only be the time you invest or a carb gasket kit. Lawnmower repair shops love home owners that neglect to run the tank dry for the winter storage.
Not knowing anything about the condition of the welder/genset, chances are 95% of being fouled by bad gasoline. So many people are unaware of gasoline's shelf life. I've seen 1 year old mowers on the road side that a little cleaning will fix.
Portable welder and power is the kind of thing that just might pay for itself the one day it is needed. A dry storage spot might be the only issue, if I had the opportunity I would find a dry spot.
Good Luck, hope it is an easy fix (cleaning)
EDIT: just have to add an FYI - Diesel Fuels shelf life is TWO Years !!!