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“You have to get a helicopter company last minute - that hurts,” Mr. Wallenda said. “There are things like that that come up that are completely unforeseen … $50,000, $100,000. These are big, big line items.”

Although the walk will be broadcast live on ABC, Mr. Wallenda said legal liability considerations prevent the network from specifically funding his preparations, materials and all the other costs that go toward his act.

He is hoping that promising thank-you gifts to donors will push up the Indiegogo numbers.

For $5, donors get a picture from the walk; for $10, an autographed poster. As of midday Tuesday, one person had pledged $5,000 for access to a VIP viewing area and dinner with Mr. Wallenda. There were no immediate takers for a private Wallenda-taught backyard wire-walking lesson, dinner and VIP viewing, for $10,000.