3, like the cost of buying the card, or the value shown in the upper left corner?
From page 10 of the rules:
Sell patents: You may discard a number of cards from
hand to gain the same number of M€.
And the example at the bottom of page 8:
Example: Stanley starts the second generation action
phase by using the standard project ’Sell patents’ (see page 10):
he discards a card to gain 1 M€