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100 mW “Input Power” does not refer to an RF signal applied to the input of the final stage. Rather, it is the DC power applied to the final stage.
As Carl stated, amplifiers are not 100% efficient so you never get OUT what you put IN. Except for some very efficient Class E output stages at nearly 80 to 90% efficient, you are lucky to get 60 to 70% efficiency.
In the FCC rule, power to the filament of a tube amplifier is excluded and I forget how the input drive signal figures in. But, generally speaking the product of the DC voltage and current applied to the final stage (not the whole transmitter) is considered the input power.
If you obtain 60 to 80 mW output you’re doing good.