Is there any formula/equation i could use to estimate the voltage induced at the input from the RF signals?
This circuit depends on the MOSFETS, in the first differential pair of the CA3130, acting as rectifiers albeit not very good rectifiers, because the fundamental MOSFET bandwidth depends upon channel-spacing from source-to-drain but the rectification depends upon the junctions of the MOSFET, such as the source-bulk isolation-junctions and not on the channel-spacing.
And the part number ---CA3130--- is a part number from the 1970s (thus 40+ years ago). To presume the same process is used today is not a good presumption.
Summary ---- you are asking about the UHF (30-300MHz) and SHF (300 to 3,000MHz) rectification efficiency of isolation-diodes of unknown doping profile and of unknown biasing condition (both voltage and idling currents).