I've finally solved it! The answer is either $0.50 or $0.98.
>What is the Cost of a book that costs 1$ plus half of its price?
We have the profit of buying the book at -$1, plus half the price of the book at $0.50, for a total profit of $-0.50, or a total cost of $0.50.
But what if the statement means "What is the Cost of a book that costs one dollar, plus half of its (the dollar's) price?"
It is impossible to buy a dollar anywhere (you can certainly trade something for a dollar, but it's not called "buying" a dollar). Therefore the only meaningful price atributable to a dollar would be the price paid by the Treasury Department to manufacture it, i.e. $0.04.
Thus we have the profit of buying the book at -$1, plus half the price of the dollar at $0.02, for a total profit of $-0.98, or a total cost of $0.98.