I remember as a teen us doing simple but fun things for each other as gifts: candy signs or candy cakes (yes, even as teens), boxes of their favorite things (snacks, fun pens/toys that aren't too childish, etc), decorate each others rooms/lockers/cars, etc. If regular gifts are too $$ but the kids are too embarrassed AT the party when everyone else is handing over a regular wrapped item, have them make something for them like a tin of cookies and give it to them the day before the party. One big thing was finding in your collection or taking a picture of the bday person, especially ones with friends, and putting them in a fun frame.
I also think it's time for them to fund part of the gift if it's that important to them -- though you could help them by paying them to do more things to do around the house than is normally their responsibility.
One thing for your DD to do would be to make jewelry for her friends. I once made a great necklace I still get compliments on that was actually a purse charm I found on sale for about $3 at Target. I just removed the big hook, added a chain and necklace hook from Hobby Lobby. All told the thing cost me about $6, and I still have enough supplies for about 2 more necklaces. I know all the hobby stores now have tons of necklace and bracelet pendants for very cheap that still look great. If I, who am all thumbs and pretty much is limited to a pair of scissors and needle nosed pliers, can do this, your DD could definitely. And I regularly see really cute (well, it's hit or miss) jewelry on MAJOR sale at Kohls. A few months ago I bought about 3 earrings and 2 necklaces for about $15. Just remove it from the backing and put it in a pretty box.