Hardcovers are books. Softcovers are just tarted up magazines.
Would I buy something in hardcover for $30 I could get in softcover for $20?
Yes, please. In fact, I just replaced my GURPS Space Fourth Edition with a hardcover after a six month long wait to find it at a reasonable price.
Taking your question isolated from anything else, of course--or at least in general. But there are other factors. For example, would you rather have two $30 hardcovers or three $20 softcovers? Are you happy spending $30 on a book like Arcane Power which has a lot of filler crunch, aka stuff you don't absolutely need, especially if you have D&DI?
It also depends upon how much discretionary income you have, which varies widely. If you're a 12-year old kid with a $10/week allowance and without the luxury of a credit card to buy on Amazon, one hardcover a month almost maxes you out but for the same amount you could get two softcovers, or one softcover and one pack of miniatures.
For someone like myself with some but not a lot of income--as I said, about $150 a month for free spending, but with other interests other than RPGs--I might not buy every supplement even if they are cheaper softcovers, but I would at least consider it. One of the Power books for $30? Even $20 on Amazon? Nope. They're only useful for character generation and I would rather go with my current plan of subscribing to D&D Insider for a month, getting the updates, then un-subscribing for six+ months.
The bottom line is that WotC wants people to buy as much
quantity of product as possible and if they are willing to sacrifice a touch of
quality in terms of production, they will and they are. And, I think, they
should.
So again, my recommendations to WotC would be:
Hardcovers - core rule books, special editions
Softcovers - splats, adventures, monster expansions
Box Sets - Starter sets, campaign guides, mega-adventures, "stuff"