I recently applied for PGWP but only being asked for $150. But as far as I know, I have to pay additional $100, so how can I pay that? Or is it not needed?
All Open Work Permit's (OWP's) are subject to an additional C$100 fee since Feb 21, 2015. It's easy to spot the fee when you're paying online, it's called 'Open Work Permit Holder fee'.
To pay it, go to the fees page:
http://www.cic.gc.ca/english/information/fees/
You're paying a fee for a 'work permit (including extensions'. Go to pay the fees online. Immigration... Temporary Residence... then finally 'Open Work Permit Holder fee'.