Smartcards are not tamper-proof (nothing probably is), they are tamper resistant, but the cost of extracting a key from the smartcard might exceed the balance of said key.
A more conclusive analysis of the cost of extracting a key would be a great help.
There has been some research on the physical security of smartcards - I couldn't find any recent studies on cost, one study from 2004 (
http://www.cs.ru.nl/~erikpoll/hw/slides/04_smartcard_attacks.pdf ) indicates a cost of $100K for such an attack. The smartcard manufacturers (NXP and Infineon in our case) also guarantee their chips against side channel attacks if a strict set of rules is followed when running crypto algorithms - that's the reason we chose to stick with their proprietary functions instead of (possibly insecurely) implementing our own.