End of lease

My wife will reach the end of her mileage allotment of 45,000 miles with 5 months to go. My options are to pay for the extra probable 8000 miles she will put on it, park it or buy it at the end not worrying about the accrued mileage. The original agreement shows a $30,000 buyout, is this negotiable and if it is by whom, Porsche or the dealer? If I do buy it what kind of interest do they usually charge?
Thanks,
Fred