How do I charge for book illustrations that cover 2 pages?

Hello.  I am new to doing illustrations.  I recently got a job at guru for a childrens book.  This is the authors 1st book.  She told me she needed 10 illustrations for a childrens picture book.  I asked her to break it down for me so I know what page each text is on and if she wants an illustration to correspond with that text.  Like many picture books, there is very little text... but there is one short sentence on every page.  There are 16 pages....  Anyhow...  After she broke it down told me there is 16 pages   ....  I don't see how 10 illustrations can work. Big question.....  If I do one illustration that is horizontal ( so that it covers 2 pages..)  Do I charge her for 2 pages or do I charge for one illustration that covers both pages......   She hasn't suggested I do horizontal illustrations... I'm just wondering if it comes to that I'd like to know how other illustrators charge.

Hope that makes sense.  Any help is greatly appreciated

Laura