If the XD is ovepriced, then many other pistols must be too.
I think you hit upon the answer there.
It's not so much an issue of supply and demand, but what the market will bear.
When the RIA 1911 and HS2000 didn't yet have much of a reputation, people weren't willing to spend as much on something they saw as a risk compared to something they knew would be 'worth' their money even though the quality was just as high then as it is today.
The RIA 1911's following grew from word of mouth and the prices were raised as people were willing to pay more. But that 2 year old $250 RIA is 'worth' the same (quality wise) as a new $350 RIA. The cost of making them did not rise $100, the supply did not decline, but the perception of quality in the public's eye had increased so the price was raised to take advantage of extra potential profit.
With the HS2000 there was an artificial marketing blitz instead of slow word-of-mouth growth.
Springfield, as an investment, bought the rights to something they knew was a good value, and then doubled the price while advertising the hell out of it. The combination of similar price, similar exposure, and SA name recognition would make people consider the XD an equal of other guns already selling at that price point and which had more established reputations.
Again, the cost of making an HS2000 did not double when it became the XD, and the supply did not shrink (it in fact exploded overnight), but the perceived value rose because of Springfield Armory's association and marketing. It didn't become twice the gun it was when a $500 price tag was slapped on it, it became a gun that doubled SA's investment.
Now, SA and RIA aren't "evil" for charging more, people seem more than willing to give them money, but realize that both of these guns could easily sell today for ~$300 and still turn a profit.
Keep that in mind the next time you see a new, inexpensive gun and immediately dismiss it as being "cheap", give it the benefit of the doubt and research it a little or you may just end up paying twice as much for the exact same thing a year later once it has a "quality" pricetag.