I just spent the past couple days doing an exercise on this theory. I took the contents list from Doug's Adventure Medical Kit Pocket Survival Pak and checked off the items I already had, which were most of them. I decided I would replace my basic mirror with the rescue Flash, and my standard compass with the 20mm button one. Those two items alone will cost me in the neighborhood of $20 including tax and/or shipping. The kit runs around $30.

So, I'll probably buy a kit, use the items I don't already have, and supplement it with ones I have that aren't in it. One thing I'm looking forward to doing is comparing the Blast Match I already have with the Spark Lite in the kit.