$300-$400 for the repair. IIIfs are cool cameras, but they really aren't any cheaper than M2s or M3s after you put the work into them. They are really old and they need to be completely overhauled. The shutter isn't the only issue. It might have a squeaky film advance and a fogged rangefinder, or worse a faded rangefinder which can not be repaired. It could have mysterious light leaks. I would realistically budget $700 for a fully functioning IIIf, not including lens.
E.g. my IIIf works great at speeds up to 100, but over that the negs are striped because it needs a new curtain. Also, in really bright light (like pictues of the sky on a sunny day) there is some kind of under/over exposure by the film's sprocket holes. So, it works, but not nearly as well as my plastic Pentax zx-m.
People have the work done and then get GDS(gear divestiture syndrome) and sell them on ebay. Last week I saw a IIIf on ebay with new curtains and a cla from Sherry Krauter go for around $350: less than she quotes for the repair. I've seen this a couple times.