You know,....its funny,..I was thinking about that very same question.
There are reasons and while you might not like them, its the reality of 2016 & beyond.
1) Porsche makes MUCH MUCH smaller quantities of air-cooled parts now. Its not just them,
every manufacturer has adopted the same "Just-in-time" method of inventory control. This forces cost amortizations over a far smaller run of parts than what they did 23 years ago, never mind what they did when your car was built.
2) Wages have not remained stagnant in most parts of the World. You are not earning the same wage you did 23 years ago (hopefully), neither does anyone else.
3) Overhead costs haven't remained stagnant either,....it costs more now for local taxes, utilities, health care, raw materials, etc.
Lastly, you ought not to forget that Corvette (Chevrolet) parts were/are made in FAR bigger numbers simply due to the vast numbers of those cars and commonality with the rest of the product line.
Any comparisons between GM products and Porsche products is invalid, simply due to production volumes of cars and parts.
Yessir,..you can certainly fit a cheaper muffler to your car, but I assure you its NOT the same thing in terms of performance or longevity. Like many other things in life, you do get what you pay for.
I'm in this industry and while I deplore the current situation as much as you do, I'm not ready to drive a Kia simply to relieve myself of the expense of ownership. Some experiences in life are rewarding beyond $$$ or words.