Apple has increased the price of the Mac mini before.
Apple will do what they want to the product design and if it warrants a price increase, they will do it. They have done this on all of their Mac product lines occasionally (and some of their iDevice products). There is nothing that stipulates that they must keep the entry level Mac mini (or any other device) at ___ dollars.
Note that the current Mac mini already has an aluminum case. Moving to a different form factor with the same approximate volumetric displacement isn't really going to change the amount of aluminum used in the case substantially, at least nothing that would merit a $100 change of the retail price. Nay, those type of price changes would be driven by internal component costs.
My point is that moving the Mac mini from the current mini pizza box form factor to something like an aluminum-clad AirPort Extreme is not so far fetched as the two products have nearly the same volumetric displacement. Apple has ample experience in aluminum case design for a wide range of products.
Anyhow, speculating on the next generation ____ from Apple is just mindless fun, unless you're in Samsung, LG or HP engineering.
Apple builds what they want to build, not some consensus product based on market research, focus groups, user surveys, etc. If there's one consumer electronics company on this planet who will release a new product with a radically different design, it's Apple. They certainly don't read the MacRumors forums for suggestions!