The said answer is Hostelz.com.
Hostelz.com is a hostel price comparison website. It is NOT a booking platform.
All they do is comparing the rates on
Hostelworld.
booking.com
Hostelsclub
and more
You will get the cheapest prices available on the internet.
That being said, Booking.com and Hostelworld.com offer the option to match-price. This means, when you find a cheaper price somewhere in the internet, just let the booking platform know and they will match the price for you.
When you use Hostelz.com, you won't need this service.