Depends on competition. Suppose you pay $300k a year in SF and the guy/gal in India's next best alternative is working for a local company making $50k which pays for a very nice lifestyle there. If you pay $100k, you're still substantially above most competitors, and the employee will be very happy.

Now with more competitors, salaries will probably end up rising close to SF ones--which are likely near to the upper bound, marginal value produced by the employee.

The biggest problem would be identifying and contacting the good devs in India. By and large they don't have access to Bay Area hiring managers, and vice versa. There's a lot of money to be made arbitraging this, hence the body shop phenomenon. It could definitely be an interesting problem to solve, but given how scummy recruiting and freelancing sites tend to be, I don't know how you would avoid an ugly race to the bottom.