Of course, when you purchase high-quality wedding invitations, you're paying for more than paper. The images, artwork or engravings that appear on your invites can't be printed just anywhere. Specialized printers are used to create embossed (raised) text and images, or an etched metal plate known as a die engraves the content onto the invitations. A letter press (a 500-year-old printing method that employs a raised, inked press to transfer an image -- similar to a stamp) can also be used, but is usually more expensive.
Printing addresses on your envelopes raises costs as well, though the expense will pay for itself in saved time (especially if she was thinking about hiring a calligrapher) and prevented hand cramps for any bride with a sizable guest list.
Excepting the homespun laser printer option, printing is pricy. Period. So make sure there's enough room in your budget for the paper you like and the printing method you prefer.