1) Have you priced other books?
2) Have you read any of the books in the series?
I'd have to agree. Irrespective of where you stand on the theology, the books are some of the worst prose I've ever read. These guys are complete hacks as writers.
Sadly, many so-called Christian novels are poorly written; even those I enjoy, such as most of Frank Peretti's works, are hardly great literature. It's like publishers figure they can slap a 'Christian' label on it and sell crap that would never sell in the secular market. Frankly I find it insulting, and more so when better-written Christian novels can't be found in Christian bookstores. (I'm thinking of Stephen Lawhead; his books are not carried at either of the two nearest Christian bookstore, so I have to buy them from Borders).
Look, Christian music was in the same place a few decades ago, and now look at it - it's great, there are hundreds of different groups out there... I'm just sad that Left Behind is doing so well, because it's sending the message of "it doesn't have to be good so long as it's Christian."