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For lex mercatoria material on fixing the contract price:
Click TLDB's "List of Lex Mercatoria Principles".
"If the contract does not contain a provision fixing the price or a method for determining it, the parties are to be treated as having agreed to the price generally charged at the time of the conclusion of the contract for such performance in comparable circumstances in the trade concerned, or, if no such price is available, to a reasonable price."
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