MobiPocket are a "wholesaler" to the other Mobi eBook stores, and they appear to have some kind of agreement not to undercut them. Consequently, they are generally more expensive than other stores. Even worse, if you live in the EU, you'll get charged the French VAT rate of 19.6% on any books you buy there over and above the listed price!
Try buying from
FictionWise instead. Excellent prices and a good selection, and they'll sell you anything.