Sooner or later, everyone bitten by Arduino bug would want to make his own extension. At the beginning Proto Shield may be sufficient.
When we have a working prototype, we might want to convert it into a series model, so PCB board will be needed.
On Arduino Forum there is a link to a very nice tutorial, how to make our own PCB board, especially for Arduino shields.
Aaron Eiche, author of the article is a member of DorkbotPDX group, which makes orders for PCBs and everyone can order with them the price is quite attractive – $5 for square inch of a double sided board + $15 for shipment (worldwide). It must be ordered in multiples of 3.