You have to question why the idea would never have seen the light of day. If it's strictly because you have a cruddy job that pays minimum wage but you are smart and capable then yeah this would be the kind of opportunity one would be able to take advantage of. If you are like many of the people here, in that we aren't making terribly low amounts of money and can save up then it's not money that's keeping you from your dreams, it's you.
I guess what I'm trying to say is it REALLY depends on the person. This isn't for everyone.
The barrier to launching is rarely money - especially these days - sometimes just having one person pushing you is all it really takes - and it sometimes is worth paying for that let alone being paid.
Seed money plus active involvement in growing the business and creating traction could warrant such a split -- especially for an idea that was sitting on the shelf. Fifty percent of something is better than zero percent of nothing.
I guess what I'm trying to say is it REALLY depends on the person. This isn't for everyone.