What is/Is there the relation between the value of a bitcoin and the prices of video cards? A significant number of Video Cards (significant investment) are required to reduce the possibility of 51% attacks. What would happen to the price of a Bitcoin if the prices of Video Cards suddenly plummeted, or (easier to imagine) their capacity quickly increase 20-fold.
Price comes from ratio of supply and demand. Bitcoin supply rate doesn't change, about 300 bitcoin per hour. So the price changes only when demand changes, and the difficulty to mine bitcoins adjusts to the new price, in another words price change would affect network difficulty, but not the other way around. Therefore video card price would affect difficulty, but it wouldn't change bitcoin market price.