If the thief were to buy the necessary equipment, I would guess that it would cost at least tens of thousands of dollars, perhaps hundreds of thousands. (For starters, he would have to drill open the processor chip's enclosure without damaging the chip itself. That would require a good microscope, a super-steady drill, micromanipulators...)  Therefore, that attack would be profitable only if the expected payoff was in the thousands of BTC.

However, the thief may be able to "borrow" the equipment from some physics or microelectronics research lab.  In that case, the thief may be willing to attack smaller targets.