Aḍḍhakāsika is the name of an especially famous courtesan, and is the price of a certain valuable cloth sent by the King of Kāsi.
Horner translates as “half a kāsī”, while Cone has it as dubious, either “made of a certain cloth from Kāsi” or according to the commentaries, worth 500. Brahmali adopts the latter reading.
All of these seem dubious. What is “half a kāsī”? We only know of “the” Kāsī, i.e. Benares. It seems hard to know how “made of a certain kind of cloth” could apply to a courtesan. “Worth 500” seems to imply that a kāsī is a thousand, but there is no basis for this.
Surely, however, it is simply an idiom, “(worth) half of Kāsī”, i.e. “priceless”. Kāsī is legendary as the home of the finest goods and products, and to say something is worth “half of Kāsī” is to say it is of inconceivable value. Compare the English idiom, “worth a king’s ransom”.
None of the renderings above, however, actually capture this sense. “Worth 500 coins” sounds like a price tag.
I’d suggest something like “priceless” or “inestimable”.