You want very cheap wedding invitations and if you are not looking to loose your style you need to think simple and be creative. Here are some fun ways to combine the two.
If your style is quirky then designing your own postcards with either a photograph of yourselves or a modern graphic design of the word invite would look great on the front, use your wedding colors for the words and the background. Use the back of the card for the wedding details - you will probably want to put the invite in an envelope so you won't need to save any space for the address. You could just post the postcard and have a weblink for your guests to view an online invite - this only works if you have young guests and tech savvy oldies!
There are a number of websites that you can design your own invites on, some are animated and then all you will need to do is email the link to all your guests. We recently received a themed invite through smilebox.com - they are an easy site to use but you do need to pay if you want to use the site lots.
There are so many free graphics programs available that using your own computer and designing your own is becoming a more realistic option. Buy some good quality paper and you will be able to print them out as well.
I have recently been playing with some free Serif software and designed a couple of simple invitations. You can download yours free through Free Serif Software the software is good, just be prepared for regular telesales calls to try and upgrade you to the fully paid version.
If I was going to pay for the graphics then I would recommend Photoshop - I find this less frustrating to use! Always good when you are planning your wedding! The link will take you to Ebay as there are some cheap prices there!
My new favorite is PicMonkey which is a free to use online graphics tool.
Of course if you are not feeling that creative here are some pretty invitation kits for under $1 an invite.