Thats a broad question! do you have a preferance for brand or how many cylinders? Triumph T140's are pretty popular and common. I work on those everyday pretty much. For for 4G it would be pretty scruffy and you would most likely need to spend another couple grand on it, depending on if its been used much or if you are capable of doing all the work yourself. Problem areas are Forks that need to be overhauld with new chrome legs, top end oil leaks, and hydraulic brakes that need going right through and faulty lights/switches and if its still got points, then 99% of people now stick a Boyer on it all of which usually soaks up a grand so or right there
real good ones start at about $8000