Keep in mind that invitations do have an important job. No matter how carefree or nontraditional you and your fiancé are, the invitation should include the date, time and location of the wedding and the reception. If you expect a response from your guests, it is also important to include a self addressed envelope with postage included. As long as these important pieces of wedding etiquette are adhered to, you can be as unique and nonconforming as you like with the rest of the invitation.
There is not a single reason to despair, even if the budget allocated for your wedding is kind of small. As I have already mentioned, there are many couples in this situation and they know the tricks they can use so as to still have the dream wedding they have always wanted. If you have no imagination or skills in what concerns creating your own wedding invitations, you can still find some cheap models in the stores which deal with such things.