Tough to answer as asked. Vices will probably vary from $50 to hundreds depending on availability, age, condition, etc. Building your bench or buying assembled? How big? Wood or metal? What will it be used for? Guys have probably put together benches on the cheap with some favors and elbow grease, or you could drop a ton of cash on a bench built by a skilled craftsman.
If I was putting one together I'd probably spend a few hundred for what I need.