We made our own invitations. My lovely groom did all the water colors and a lot of the layout. I did a lot of the wording and some graphic design. We used PhotoShop to make the images and a desk top printer for the printing. They were one of the things we got the most compliments on.
I ordered the paper from here: http://www.treecycle.com/catstationerycards.html. We used Genesis™ Recycled Stationery in Milkweed 8.5 x 11 80# cover. I recommend paper that is at least 80# . That is the minimum thickness the USPS will take for postcards. Becareful, they also have a minimum size.
We tried not to use much paper. We did this by making the RSVPs and Rehearsal Dinner invites postcards. It also cuts down on the amount of money you spend on stamps.
For the wedding program I made, it was a 8.5x11 page that was folded in half length wise. It looked really slick. Although I do have to admit some of the font got a little small. A lot of people commented on the quiz on the back of the program. It gave people something to talk about before the wedding started. We had a friend who's got a good sense of humor stand up during the toasts and tell the answers.
$73.90 Stamps (100 Invitation, 150 Postcard, 80 Thank You)
$84.00 Ink ($12 per color cartrige)
$26.99 Card Stock (500 pages of #80 paper)
$30.17 Envelopes (250 matching card envelopes)
Total = $211.06