You could always have the prize be a free 500 point army!
I've been mulling something like this over, and it's a sticking point.
A Daemon list...
Herald of Khorne
Herald of Khorne
Herald of Khorne
Herald of Khorne
Nurglings
Nurglings
Soul Grinder
Retail value, over $200.
Carte blanche would be difficult to keep reasonable. I can't even imagine the damage IG would rack up given carte blanche. I'd have to chat with Chase about an event's financial expectations.
There are a few solutions I have in mind, in no particular order.
1. Winner take all. Pro: A bigger prize likely to draw more players, winner gets to pick what they like. Con: Only one prize, increased likelihood of bitching and grumpiness.
2. Pre-picked 500 point armies. Pro: Finances are dictated, more prizes. Con: Maybe you don't want one of the choices. It would be a bit of work, but perhaps with an option like this, 500 point lists can be for every codex.
3. 500 point army with an established monetary limit. Pro: Best of both worlds. Con: Some choices and armies are more expensive than others.
4. Straight store credit, homie.