£8-10k may be a conservative price, I have done a trial of this repair on a 182Q airframe, and it was more work than the service letter suggests. But, when I began the installation of the kit, I found the crack in the underfloor bulkhead which is the concern of the service letter and AD. And, this airframe had only 3300 hours TTSN. I have spoken with the FAA Engineer who drafted that AD (who knows her stuff, as she used to work as an engineer at Cessna), and she reported that there is a suspicion that a factor in this strain on the fuselage is simply the airplane being parked in the wind for decades. A 182 in Wyoming was reported as having the cracks, even though its airframe time was low. But, it had sat tied down, rocking in the wind its whole life.
The strain which causes this cracking is the relative motion of the wings (cabin top) to the landing gear (floor). So in reality, flying the plane gently is about the least straining thing to do to the airframe. It's noteworthy that any floatplane version of the affected types will have crossbracing in the windshield, which is there to resist this straining during rough water operations (no suspension). But even a landplane which had been operated on rough ground a lot, or even sitting parked in the wind, can cause this strain.
The AD does not affect 170's, 180's, 185's and early 172's and 182's as they have a different structure in that area. Similarly, 150's and 152's are not affected, ans their wingstruts mount differently. But, out of an abundance of caution, I did remove and inspect the wingstruts and mounting areas on my 150M during the spring annual inspection. It's not affected by the AD, and it's comparatively low time, but it's still an aging aircraft, so deserving of the additional inspection. But, for the affected airplanes, it is a very valid concern.