Depends on what you are looking for. I mean, if you are looking for a Shelby, or any Boss then the prices are just freaking crazy.
For a good mustang say a 65 or 66 coupe you could get one for say 5k to 10k US. Covertibles go for more of course.
And if you get a Cali car then you don't have to worry about too much rust. Although, I think with your MOT checks over there, some of the cars running about on US streets would definitely not be legal over there!!