Um, probably none. I'd like to give Corvettes away, or just have one, but since I can't afford to do either of those things, and nobody is likely to give me one or donate theirs to the cause, it's a pretty unrealistic idea.
Even sites like Tom's Hardware, Anandtech, RealHardTechX, HardOCP and TechSpot can't afford to do that, even with their manufacturer ties and corporate backing, so I doubt this is something that even the most well intentioned of individuals would ever be able to accomplish. Plus, you have no credibility, so it's further unlikely that anybody would have the necessary trust factor to just GIVE you anything. Good luck though.
Some people have started selling houses by raffle in the UK, smallish ticket price, £2 to £200, and a large number of tickets, someone gets the house, the sellers get to move. Excesses often go to charity.
The point being it's a good idea to own the thing you want to raffle off in the first place.
So you could bootstrap it: build a cheaper one, sell for $10 tickets, sell 100 tickets and you've made enough to make you next PC better. Better odds than the lottery. Start shaving off 10% from the top and you'll have covered your first build in 5-6 builds.
It's not that mad an idea. But there will be laws and regulations that cover it.
And also, like I said, you have to have some kind of credibility. People are unlikely, or at least they should be, to just give any ol' body, their money without having a reason to be able to believe they will follow through on doing what they say they will do. There are many examples, recent ones even, of people getting ripped off in a major way by supposed investment opportunities.