Saul,
Railroad watches don't usually have covers but are open face. As for what you'd pay, a good round number would be $500. It would depend on how old it is, how broke it is, how rare it is, etc. You could buy an old broken one and have it fixed but it would depend on what's wrong with it. Just a new spring or something would be fixable based on your financial situation but for most people would be fixable. But extensive work is probably not worth it even if you could afford it. The trouble is, you buy it and then discover there's too much to fix and have to scrap the idea. Now you're stuck with an unfixable watch. A repaired watch could be over $1000 but I'll stick with $500 as a good round number to base your expectations on.