This is fairly dense academic writing, and a full answer would probably require reading other things the author has published to see if they have defined 'debased area' more clearly in previous works, however:
I believe "debased area" here is a reference to a specific, historical region where debased coins were produced. A debased coin is one which has its value reduced, often by reducing the amount of precious metal it contains.
Sanabares was a king, and during his reign coins were minted (produced) in this area, just as they were during the rule of the Gondophares dynasty.
These "debased" coins include imitations of:
(coins of) Hermaios from the Kabul Valley
the Gandhara coinage (after Azilises)
(coins of) southern Arachosia
Why "debased area" rather than specifying a place like "the Merv-Herat"? Possibly because the precise location or correct name is not known, only that there was a common location where these imitation coins were produced.