Nanobots tend to be portrayed in popular sci-fi as being mechanical (as in super miniature robots), but the reality today and foreseeable future is they're biological--think reprogrammed virus'. The difference is their task/s are the designer's rather than to reproduce and consume the host cell.
Yes, it would be great to have them fix and do things, but the expense will not be trivial--and not mainly due to the tech, but due to the insurance and law folks.