Having had some lengthy discussions with some family members who are doctors/nurses, I've come to the following conclusion. If your injury is life-threatening, the best you can do is carry a satellite based beacon to request immediate aid. Otherwise just drive yourself out of there.
I mean, I carry bandaids and painkillers, sunscreen, aloe, etc. If there's a serious wound there's no real difference between using a tshirt and duct tape vs a clean cloth bandage and medical tape, except that one costs a lot more.
That said, there are some nice 1st aid kit "builds" in this forum, so if you want to spend money you'll undoubtedly be able to. A nice case, stacks of varying sized bandages, quickclot, splints, nice trauma scissors, etc. You can spend over $500 very easily. If you have a real emergency though (pulmonary edema, myocardial infarction, stroke, etc.) none of that will do any good.