Info Value = the added accuracy the markets provide relative to other mechanisms, times the value that accuracy can give in improved decisions, minus the cost of maintaining the markets, relative to the cost of other mechanisms.

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Robin Hanson:

A highly accurate market has little value if other mechanisms can provide similar accuracy at a lower cost, or if few substantial decisions are influenced by accurate forecasts on its topic.

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[Here's Robin Hanson's website. For your information (if you are a newbie), Robin Hanson is the most advanced researcher in the field of prediction markets. He co-invented the modern-day prediction markets, the concept of decision markets, and a new marked design, the Market Scoring Rule.]