It depends on the people. I'd make a minimum of 5 bucks, and plan to have less people want to get the wallet. People don't know what bitcoin is, so by the time you get explaining to them what a crypto currency is, and how to obtain a wallet ect, you will end up having much less time per person than you plan. A lot of people will come by to talk to you, or look from afar, without any intention of getting free coin, go engage these people on the fence, who may not have a phone. Likely the people who will benefit from this may not even have a smartphone.

I am 31, I don't have a smartphone. If you came up to me, and your pitch was giving me something free on my phone, I would walk by you. So, you have to use the giveaway as 1 tool in your arsenal. 500-600 dollars in giveaways may not be worth the entire 5 hours time slot. Economically if we where a business and had as large an investment in the booth as we did, we would have to beef up the advertising budget significantly to make the numbers work. You MUST use that time better. To spend 5-20 bucks a person when giving them money to lock them in, I think is a good way to do it.