RedK already pointed out that you're not comparing equivalent machines. A Mac mini is more like a Dell Studio Hybrid, not an Inspiron 545s.
As for your chosen upgrades to the Mac mini, this is why I said "pick a reasonable Mac". Maxing out the upgrades will never result in an ideal price for anything (Mac, PC, or whatever).
For example, it costs $150 to go from a 2.26GHz to a 2.53GHz Intel Core 2 Duo. Is the slight speed bump really worth the extra cash for the average user? I don't think so. Upgrading the processor on that machine IMHO falls into the "blindly buying the fastest model available" category.
Another example with the mini, upgrading to a 500GB drive costs $200 from Apple, but a new 500GB laptop drive only costs $79 on Newegg. Grab a screwdriver and do the upgrade yourself. Not only is it cheaper, but it's also more fun