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Explanation: चेम्बर्स डिक्शनरी के अनुसार बिल्यॅन एक कुधातु है; (चाँदी, ताँबा, टिन आदि की) मिश्र धातु. फ़ॉदर कामिल बुल्के ने अपने अंग्रेज़ी-हिन्दी कोश में इसके लिए अपधातु शब्द का प्रयोग किया है.
as you mention, billons are minted from debased metal, so the coin is, in effect, a debased coin (or debased-metal coin). In this case धातुत्तहासित सिक्का should be regarded as an apt translation, in my opinion.
Billon Coin isa
small-denomination devalued metallic coin whose face value exceeds the value of its metal content and its minting expense. It is a replacement for gold and is, in the final analysis, a sign of value. Used mainly in the retail trade, the billion coin fulfills only the function of a means of exchange and, to a limited extent, of a means of payment. Since 1933 only billon coins have been put into circulation by all countries.
Billon coins are minted from low-grade silver and from copper, nickel, aluminum, and other metals. The minting of billon coins, in contrast to minting coins of full value, brings the state a monetary profit and is a closed minting—that is, it is produced from metal belonging to the state. The release of billon coins is limited by the exigencies of monetary circulation, and their distribution among coins of various denominations and their maximum wear limits are decided on the basis of practical experience.
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35 mins confidence: peer agreement (net): +3
billon coins
धातुत्तहासित सिक्का
Explanation: धातुत्तहासित सिक्का means "debased coin", which I think is pretty close to what you have in mind.
In this reference 'billon coins' have been called 'highly debased coins'
The translation workplace 
peer agreement (net): +3