Everyone should have at least some sort of first aid kit when out in the backcountry. Since I'm a bit of a medical geek, I tend to carry a wee bit more than others. I personally have 3 kits, a day hike/solo backcountry kit, a multi-day/SAR kit, then my full on jump kit that goes in the car everywhere. Since opinions vary on what a good first aid kit contains, I believe in either building your own or revamping an existing kit. The REI kit is from the Outlet, spent like $24 on it, then added a few items for a light kit. The Conterra kit was just an organizer that sells for $33, and all the contents were added by me. It fits nicely in the outer pocket of my pack, along with a BP cuff and stethoscope. The jump kit was a $120 pre-pack kit that I went to town on. Probably has twice as much in it than I bought it with, but since it stays in my car, its stocked more for "car accident along the way" than carrying up the trail. I've looked at Adventure Medical kits, and think they are well stocked and at a good price. If you want to build your own, I suggest buying a pre-made kit and modifying it to fit your needs.
http://survival.outdoorlife.com/blogs/survivalist/2012/04/10-most-important-pieces-gear-your-first-aid-kit
http://www.outsideonline.com/outdoor-adventure/outdoor-skills/first-aid/First-Aid-Kits.html