Why are IaaS hosting services are usually fixed-priced and PaaS are usually dynamically-priced?
Fixed price tiered billing is an older billing model that suits a monthly billing cycle where usually the level of resources provided does not change over a month. Even if it is changed, this is usually met with friction from the billing department in form of the higher pricing tier being applied to the whole month.

By comparison AWS can be credited as the first provider to offer simple building blocks on an hourly basis (now per second). This allows for much more sophisticated usage of resources in form of:


auto scaling to the best amount of resources required.
turn services off out of hours or until turned on manually.
ultimately to offering functions as a service, the most finegrainerld resource billing usage available.


So I don't see these two items as being directly comparable, but one is a progression from the other.