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Two thoughts-----
Upgrading the mac mini to HP specs would cost much more than the bench price of the first comparison you used. (Thus apples and oranges)
Second, the Mac is a cost comparable, and superior and cleaner, operating system for the creative computer user because it includes standard software that is usable and desirable (as opposed to the come ons loaded on PC's), which balances out what I would need to spend on software for a PC.