I don't think it's as simple as this.
Smaller Screens equals
Less power usage which equals less battery needed. In addition to being cheaper to manufacture.
Smaller and more inexpensive iPads mean more impact in emerging markets where $400 bar of entry to iPads isn't going to fly.
Apple doesn't really have to worry about cannibalizing their iPads. The iOS ecosystem nets them 30% revenue so there's a financial incentive to keep people buying apps and making the overall market larger.
Look at it from the mathematical perspective: volume is a cubic function, so dropping the size of the device in all 3 dimensions results in a significantly smaller internal volume. You still need to engineer the device to squeeze everything into an appreciably smaller volume. The screen is smaller and needs less battery, but the volume is reduced so you can only squeeze so much battery inside. The task is to fit everything that's in the normal iPad into a smaller cube and still maintain the desired level of user experience. This is not a "well just do it the same, but smaller" exercise. All that re-engineering is very costly because you're starting from scratch. And bear in mind, getting everything to fit inside the current iPad was no cakewalk either.
As I, and smarter minds elsewhere, have noted your best hope for a cost savings is about $60-$65 in BOM/manufacturing costs.
The "getting people to buy more from the iTunes/App store" is a non-starter. Apple doesn't
net 30%; that's a
gross figure. They don't keep it. It costs money to serve, vet, organize and present that data. They don't post profits from their content sales but it's likely under 10%. And they sold less music/video/books/apps from their online stores
in all of 2011 then they sold in iPads alone just last quarter ($6bn plus in iPad sales vs. about $6bn in combined sales of content all year). 30% of the iPad sales (nearly $2bn) is profit. So before Apple decides to offer a lower-priced, lower-profit smaller iPad it needs to clearly evaluate how it will eat into their existing iPad sales structure.
Bit getting people to buy more content is not a great business plan - perhaps it works for Amazon (where every new penny earned is a new penny to the company), but not Apple.