The problem has quiet a few information's that need to be checked first. These information's will help to determine the required answer.
Actual price of the card = $4
Price of the card calculated by Dennis = $4.50
Amount of error made by Dennis while calculating
the cost of the card = (4.50 - 4) dollars
= 0.50 dollars
Percentage of error made by Dennis while
calculating the cost of the card = (0.50/4) * 100
= 0.50 * 25
= 12.5 percent
So 12.5% error has been made by Dennis while calculating the price of the card that he wants to buy for his wife.