I made my invitations myself, cost me about $50 all up for 45 invites (plus postage).
One tip I have is to include self-addressed envelopes already stamped - that way there's no excuse for people to not RSVP (oh, I forgot a stamp. Oh, I forgot your address... etc).

I did 2 parts to the invites - the first was the formal invitation, and the second part was a single strip of paper with a map to the venue, information about accomodation in the area, our wedding registry information and then a "cut here" section for the RSVP. I did it all on the computer and I got 3 strips out of one 12x12" scrapbook paper. For the invites, I got 6 invites out of each piece of card and 3 invites out of each a4 piece of vellum. Just 1 night of playing around to get it looking good, then another night printing out everything, and Mum & Dad assembling the invites for me.