First of all? Who is talking about low prices here?
It's apple. You don't buy an apple because it's the lowest price. I'm pretty sure most people agree with that statement. The fact that there is 2 types of 13 inch laptops (mb and mpb) in the lineup in the first place shows that there is a market for a "better" 13 inch. If it's just lowest price then you buy a MB.
Really? So people don't care to spend an extra hundred dollars (compared to the previous generation) when looking at a MacBook Pro?
Think about it. People love the aluminum design of the MBP. They want one. If you increase the price by $100-$200 to include better specs, then many would opt not to buy one. And since they don't really dig the look of the white MB, then they might go look somewhere else (Dell, HP ,etc.).
Instead, if you offer them a nice looking aluminum Mac, which has all the processor power to handle what they need and more, and you offer it a reasonable price, the product has just become much more probable to be bought.
So yes, people buying a Mac have decided to spend a few extra bucks to get one, but there IS a limit were it just becomes too much.
To put an example. In my college the 13" aluminum MBP has become extremely popular. You don't see many white MBs. Why? Because people want to have the "Pro" aluminum Mac, the one that looks cool. They are willing to pay the extra $200 for going from the white MB to the MBP. But they might not be willing to pay an extra $100-$200 (from the already extra $200 from the MB).
I know, $100!! Come on, it's not that much! But the truth is that in your mind $300 do start to feel like too much. So I think Apple is making an excellent decision in offering an accessible MBP, which has much more power than most its user would ever need. They are obviously loosing some buyers, like some of the above posters, but my guess is these buyers are a very small percentage.