Is it cheaper to manufacture the discs this way?

According to DVD Town, it looks like the DVD Forum will be bringing out a "one-sided HD-DVD Combo disc later this year."

The plan is for the disc to have have three layers. Two of the layers will be devoted to DVD with the remaining one to used as the "HD-DVD layer."

A user will be able to insert the disc into their player and depending on the type of unit they have the movie will either play on standard DVD or HD-DVD.