Technically, the question can't be answered, because you did not specify the number of cards. If the game is played with a standard deck of 52 cards, you need to say so in the statement of the problem.
I will assume the cards are a standard 52-card deck....
Use the basic definition of expected value.
probability value (winnings
minus cost of playing)
-----------------------------------------------------------
winning card 2/52 -5+20 = 15 (2/52)(15) = 30/52
other card 50/52 -5 (50/52)(-5) = -250/52
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expected value: -220/52
The expected value for playing the game is -220/52 dollars, or about -$4.23.