wow.... umm Ok. There is a serious lack of necessary information here. But that's no problem, we get that all the time. We would need to know what model, what condition, what the final goal is, etc.
If it is a treadle still it uses a Leather belt (most repair shops still carry them)
"repairs" could cost anything from $10 to $1000 depending on what you repair and how and in what part of the country... $10 is for a belt, $1000 is for a complete like factory full restoration of machine and cabinet
Not trying to make it sound scary, but without real info it is hard to guess.
If you look at the "sticky" threads here (the ones that stay at the top) there are some GREAT write ups on how to clean and restore a machine and cabinet yourself, not too hard, very inexpensive, much fun. Everything I discussed above can be done for $50-75 if you do it yourself.
Let us know if you want some guidance, and please post pics so we can give you some real advice!