That and, what's the price difference between a USB card reader + supercard with screen and such vs the cost of the cheapest BTC runnable smartphone?
I imagine the "supercards" are less than $20. The point is that once you have a display and keypad, you don't need a reader. So the cost is similar.
you could just input the price they display to you on your card, enter your pin, then hit send. The card signs a tx with only that exact value so they can't change the price after you begin entering your pin. It could be doable but it seems a bit complicated imo.
No, there's no need to manually enter the transaction amount. Transactions can be sent over the wire. This is irrelevant.
assuming you can even implement a protocol that doesn't allow the private keys to be leaked
A lot of smartcard apps are poorly designed. But it isn't black magic or anything. It's definitely doable. Look at the satellite TV access cards. They can be reverse engineered, if you have access to the card itself and a scanning electron microscope.
My question is, even if you load BTC to your smartcard's address without connecting it to your computer somehow, how does the card know how much is on it?
The essential usefulness of SmartCards was that it allowed for the writing to and reading of data (account balances) from the smart chip even when the merchant didn't have access to the card payment networks i.e., when the phone lines fail.
Why are you mis-quoting me?