Like most questions it is not possible to give a definite answer to this question. Regarding ``lo-fi'' stereo components like modern Mini systems or ``mid-fi'' components like small amplifiers, tuners and so on ranging in price up to 500 US$ you must strictly deny it: any DIY project would be much more expensive because this type of stereo equipment is produced very cheap in very large quantities.
This changes however if you look at components which are made in small numbers where the price of the raw materials is much less than the price you have to pay to a decent dealer. This is especially true for the so called ``highend'' equipment-often the ratio between the dealer's price and the raw materials is about tenfold or even higher. Just have a look into the innards of a preamplifier and make a short calculation of its sparse contents. Also this equipment is usually very beautifully made and built to last, with nicely decorated front sides and gold letters all over the place. This increases the costs of production even more and is something the DIY maker need not to do-a preamplifier sounds the same whether it sits in a regular 19'' housing available at your local Radio Shack or in a platin housing with quarter inch golden front plates. Therefore it is possible to make very good highend equipment by yourself without paying much money as long as your time does not count.
Furthermore DIY of stereo equipment has several advantages: fine tuning of the electronic circuits allows the adaption to your personal taste, and you do not need to buy new equipment after some time due to new techniques-just replace old fashioned parts with modern ones to bring the component up to date.
The most preferable and thankful DIY projects are preamplifiers:
There were several reasons that eventually lead me to make my own preamplifier:
My thanks go to Thomas Magin, who converted my manually drawn schematics into CAD format so it is now possible to make a pcb. Furthermore I have to thank my (former friend and) wife who missed my company for many evenings and weekends and instead had to survive uncountable listening tests.