Polybutylene plumbing has a long and sordid history of leak problems. It was originally developed in the 1970's and promoted as a technology that was easy to install and less expensive than copper. Unfortunately by the 1980's, many structures started reporting leaks and upon closer examination, it was found that the interior surface of the pipes as well as the metal or plastic fittings were breaking down and flaking apart and the polybutylene plumbing needed to be replaced.
The best thing we can tell you is that it is only going to get worse. Therefore, the sooner you can get the PB pipe replaced, the better. If you are between tenants and have the money to replace polybutylene plumbing, now seems like the perfect time to get it done. The cost is going to be directly proportional to the amount of work it will take to get to the areas where the pipes are, as well as the cost to repair those areas once the plumber is done with the replacement. As with all home improvement jobs, choose your contractors carefully and be sure they are insured before letting them step foot inside your home to replace the polybutylene plumbing.