The designers charge a flat fee for a customized design that you get in a PDF and then you have a variety of choices with printing options. You can go through a company like Snapfish and order 4x6 prints on photo paper for as low as $0.07/card, or you could do cardstock from Vista Print for as low as $0.28/card.
*For envelopes...check out Envelope Mall. You can get 100 for around $12.
Happy card shopping!