As the base game goes, there are only 2 cards that gives you the option to BUY the card or discard it. And those 2 cards clearly say 'BUY or discard'. And by buy, it means pay 3mc to keep them in your hand.
There are a also few that say pay X (money or energy) or discard a card to draw. They're clear about the up front cost, but you pay before you draw and the new card goes into your hand whether or not it turns out to be one you want.