There are gold, silver, and possibly copper coins in Middle-earth. There is insufficient information to form a valuation system, however. I've scanned some of the replies you got and have to agree with Martin for the most part.

If you'd like to know where money is referred to, check out "A Knife in the Dark" in THE FELLOWSHIP OF THE RING. Barliman has to compensate people for the loss of their horses and ponies and he pays in silver pennies. In the story about Thror's death Tolkien wrote that Azog threw out a bag with a few coins of little value to Nar, Thror's companion.

Bilbo lugged back a couple of bags of gold to the Shire -- presumably there were gold coins in the bags. There is indeed an Elvish word for "gold coin", kulusta.

I would say the suggestions you're getting from the others are your best bet.

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Parma Endorion: Essays on Middle-earth, Revised Edition