You have a one in a thousand chance of winning the 500 prize.

So if someone gave you a ticket, your expected return, if there were only the one prize, would be 500/1000 = 0.50

Now, if there's a second prize, say 250, the chances of you winning that particular prize are 1/1000, so your expected value from that prize alone is 0.25, BUT there's also the chance that you might win the 500, which has an expected value of 0.5, so between the two prizes, you'd expect to get 0.75.

Similarly for the 100 and 50 prizes.

Giving

500* (1/1000) + 250 * (1/1000) * 100* (1/1000) + 50* (1/1000)

= 0.9.

But you have two tickets, so you're twice as likely to win (approximately), so your expected return is 1.80.

But, if you paid 2 quid (total) for your two tickets, then your expected return is 1.80 - 2 = -0.20.

Perhaps the question in the book is phrased ambiguously?