Well, by definition being 'cheaper' has a hard upper bound: you can't get cheaper than free, so the total gain is limited to the price of the more expensive product. (If Dropbox costs you $100 a month and Drive drops to as cheap as possible, $0, you only gain $100.) But if they pay you, there's no such limit (perhaps they'll pay you $200, or $2 billion).
There isn't. Apparently some HNers don't understand how to reason about economics.
It's conversational English. Consider applying the Principle of Charity. He meant "Google would need to pay me to use the service in order for me to switch.". This doesn't even explicitly mean that. It just means "I don't find this offering compelling.".
See, we could have avoided this whole thread by applying the Peter Principle in conversation.
> What's the difference between paying you to switch, and just being vastly cheaper?
Time value of money.
I think what he means by that is "even if it was free, I still wouldn't switch". And from free the only way they could make the price even better would be to pay him.
His point was that it could not be cheaper enough without going negative.