You want the best and brightest. It seems he's using this money as a metric for determination, but I suspect it'd be more a discriminator for wealth. I think Paul's idea of allowing founders in and his terms suggested sound like a better predictor of determination.
I guess only organized crime would profit -- provide the money and then blackmail everyone. The immigrant whose whole family is in giant debt, the family... For US it would mean communities of hopeless immigrants from poor countries, entrapped by a vision of better life, held as captives by a mafia. This already happens, and it could only worsen.
What would happen in a system like this is that the employers would pay for it. If GE, IBM, or Microsoft got the chance to bring in people like this, they would take it.
H1-B Visas aren't exactly what's being proposed because there's still a limited number of them each year, but there is a cost to the company for sponsoring someone.
I think $50K to be legal, and not even needing to come up with the money up front, but instead being given a loan, would be considered a bargain.
Consider the Philippines - current population is 90 million; thus top 10% of people (who will have money) is 9 million. Given the wide disparity in income, it is reasonable to believe that it would be trivial for any of those 9 million to raise $50K.
Do you really want to allow in every rich kid in the world that thinks it'd be cool to screw off in the US for a few years at the expense of someone who's distinguished themselves in their field?
Also there would be some sort of screening for criminals.
But in a sense you are right, such a scheme would lock out the foreigners who have most to gain from immigration.