Being a US citizen you are automatically given a 21 day visitors visa.
To extend your stay by another 38 days to 59 days (21 + 38) the rates are as follows:
P1,000 - Visa Waiver Application Fee P 500 - Visa Waiver P 20 - Legal Research Fee P 500 - Express Fee P1,010 - BI Clearance P3,030 - TOTAL
The above information was taken from the Philippine Bureau of Immigration website. The link is noted below, you may have to copy and paste the link into your browser.