I made my own wedding invitations (and programs and place cards, etc). I never priced out the difference because it was never an option for me to buy them - I never saw anything I even remotely liked. That was over 5 years ago, and I would probably do them differently now because I have about a zillion more tools and supplies, but they were pretty basic and simple. I did have a small wedding - around 75 guests, I can't remember exactly how many invitations that worked out to be. The theme for the wedding was seashells, and I found a seashell print vellum that I printed the words for the invitation on, then stamped 4 seashells in navy and silver on each corner of cardstock and tied the two together with ribbon. The only kink in the plan was I could not tie a bow to save my life, so one of my bridesmaids had to tie them all! At the time, there was a local stationery store that had envelopes in bulk fairly cheap and I printed all the reply cards, return envelopes and outer envelopes myself on my regular printer. I was definitely glad I did it myself because it could be just as I wanted it to be.
This has already been mounted in my scrapbook so its kind of hard to get the full effect, but heres a picture: