Currently Africans are commonly paying over 10,000 euros for an illegal trip to an undetermined European country. The families raise the money for young men even though there's no guarantee whether the kid will end up in Norway or Italy (and often no further than Morocco). Compared to this, a five-year US visa for $50k would be an extremely attractive option. I doubt that desperate young men whose families are hopelessly in debt are the kind of immigrants that the US would want to specifically attract.
No one should ever be able to buy their way into a country with raw cash. Skilled immigration policies that aren't stupid (i.e. H-1B is the dumbest "immigration" policy I've ever seen) are a much better idea.
Then you, too, support reform of the existing US immigration laws (see Kirk's comment above -- investing $1m will get you in).
That being said, the whole idea still sounds sort of sketchy to me. The US (like all industrialized nations) doesn't really need capital - not $1M at a time anyways. What it needs is skilled labour - things that will help the country compete internationally. IMHO skilled labour really ought to be the only form of immigration.
The tax structure in the US is progressive. If we accepted unlimited immigrants we would end up with many low skilled workers coming here. Regardless of whether this is desirable (I think there's moral arguments to be made in its favor), it's politically impossible.
Placing a bar that involves a fix fee per year, if the issues related to being enslaved could be avoid, would attract exactly who we want -- those who would do so much better in a developed country that it's worth while.
I see it as if you were born in the US, you were freely given this thing, the right to live in the US, that is very valuable on a world market. Why not allow others to buy that same thing?