Back-to-school notebook sales were as strong as ever this year but skewed in favor of cheaper systems, the NPD Group said today in a new study. About $1.6 billion of the $7.6 billion spent between late July and mid-September went directly to notebooks, or about as much as was spent last year despite ongoing economic problems. However, the revenue came at the expense of system prices: while more systems were sold, the average price of a school notebook fell from $804 last year to $624 this year.