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.
Perhaps there is a hobby that both of you love to do. You can use this 'theme' in your wedding invitations as well. If you both love to fish, you could use fishing line and lures for your invitations, or a hobby of wood_working could provide you with a way to expand your hobby into creating some special invitations. Regardless, if there is a hobby that the two of you share that helps to define you as a couple, don't be afraid to use it to your advantage and choose the best part of your hobby to direct your unique wedding invitation choices.