It is/they are?

[Winwin2011]

1)The book is $13 and the magazine is $5, so it is $18 altogether.
2)The book is $13 and the magazine is $5, so they are $18 altogether.

I am wondering if it is correct to use "they are" in sentence 2.

Thanks for your help.

[emsr2d2]

1)The book is $13 and the magazine is $5, so it is $18 altogether.
2)The book is $13 and the magazine is $5, so they are $18 altogether.

I am wondering if it is correct to use "they are" in sentence 2.

Thanks for your help.

An argument could probably be made for both.

The book costs $13 and the magazine $5, so the total price is $18. ("The total price" is singular so "it is" would work.)
The book costs $13 and the magazine $5 so, together, they cost $18.