The Long-Stay Visitor Visa Extension (LSVVE) will cost you either P13,900 or P11,500 depending on your nationality. It will be the lower cost for almost all Western countries. I you can currently only get this visa in the BI Main office in Manila. Link: BoI - LSVVE
I think it must be more than that as a 2 month extension is already 4100 PHP?? I can't imagine the Immigration Bureau giving a discount
Not all extensions are the same price. The first 2 months is the most expensive. After that the prices go down a bit.