Para dropping is another area where it is/was common to get paid even if on just a PPL (no CPL). Especially in Spain or somewhere where the wx is nice so you fly all day.
No cheaper way to build hours than to get paid for flying.
That's what most PPL instructors do after all; they need to build 1000hrs somehow; then they get an airline RHS job and that is then enough for an ATPL which is 1500hrs of which 500 needs to be in a multi pilot plane.
Cheapest way to build hours towards a CPL has to be renting a C150 in Arizona.