User micgoosens already runs price prediction competitions. Here is his latest one with around 100 users entering: https://bitcointalk.org/index.php?topic=5096275.0. What's more, they are free to enter and he provides all prizes himself - for this one, he is providing a prize of 0.125 BTC.

I'm sure in you started a thread based on your idea, you would get a reasonable number of entries.