We used Lantz diy cards, you can buy blank / empty cards in batches of 6, and then using your printer print off the pages to go inside the cards and just glue.... you can get matching invites / reply cards / thank you / mass cards, etc. We got them in their store, but i believe they also sell them online (i just wanted to compare differnt products).
I reckon for 100 people invited to our wedding our total outlay was about Eu130.... that was last November.
No-one will think your cheap, most people know that the cost of wedding are huge!
Another option is M&S, last 3 invites I got were from there!
Best of luck