I searched Pinterest for DIY Thank You card designs when I saw an errant post on DIY Bookmarks that I loved (as an avid reader who has a naughty habit of folding pages). They weren't using them as cards, but I kept thinking to myself... Why not? There's plenty of room for your message on the back, You can use scrapbook material on the front. It's extremely frugal, and we're saving trash (twice, since the bookmarks are a third the size of a card and the recipient can actually reuse it if she has a need for it).
I created these bookmark cards using scrapbook paper that I already have, and bits of leftover ribbon. Since I have TONS of scrapbook paper organized by color, but not a lot of ribbon, I picked a ribbon I liked first, then matched it to a coordinating scrapbook paper. I kept running out of ribbon for different colors, so I would just switch to a new color combination. Which actually worked out well, because I could choose a style that I thought the recipient would like best. The only thing I had to buy was blank paper bookmarks. I had originally planned on just cutting them myself from white stock paper using my paper cutter, but I ended up at Michael's Crafts when my husband decided that he wanted our fourth of July holiday to include him teaching me how to silkscreen baby onsies (That's right, my 6'3'', 260 pound, bad ass husband silkscreens... and sews, bakes, creates fancy french dishes that I won't eat because I can't pronounce the ingredients, and knows every single line to the Broadway shows Rent and Legally Blonde: The Musical). Michael's had an enormous pack of blank paper bookmarks for 3.49. Sadly, I threw out the outer package before I saw how many were included, but I'm going to make an educated guess of 150. I grabbed that because $3.49 was well worth the time it would take me to correctly measure out 30 bookmarks.
When I got home, I used my corner rounder on each side of the book marks, Then cut strips of scrapbook paper (I just eyeballed it to be slightly smaller than the bookmark). I rounded out each corner of the scrapbook paper as well and glued to the center of each bookmark using scrapbooking glue. I then used my small hole punch to punch a hole in the top, inserted ribbon and tied off. It took about 15 minutes to do all 30, but the majority of that time was spent deliberating on what combinations to use.
I paired them with stationary envelopes that I already had and made a bundle to fill out when I have some down time. The next day, I wrote my messages on the back of each card and included a tag line that said "*Feel free to reuse me as a bookmark!"
Instead of spending the $9.99 on 36 thank you cards that I had planned, I spent 3.49 for (guesstimate) 150 thank you cards and other projects. Plus, nothing says thank you better than giving someone something they can use.