I added leading edge aux tanks to my -7 (not yet flying) by following the example of some -9 builders that *are* flying. One of them is an engineer, so that helped with the comfort level.
I made the middle bays of the outer leading edge 'wet' using the stock components, leaving the outer bay and the bay next to the main tank dry. I added stiffeners to the bottom of the leading edge skins, similar to the original tanks, and covered the lightening holes in the spar to use it as the back baffle of these aux tanks. Calculated capacity was a little over 12 gal per side based on 'per inch' numbers for the stock tanks. It now appears that I have a bit more than that; I think I forgot to allow for that extra space between the stock baffle & the spar. Total added weight, including all fittings but not including the tubing or additional fuel selector, etc, is about 2 lbs per side; basically the weight of the quick drain, fuel cap, extra aluminum to close the rib holes and spar holes.
Cost, IIRC, was under $200 total, but was a *lot* of extra work, because I was basically making design decisions as I progressed (no plans to work from). I didn't change rib spacing because that's worked for others, and the aux tanks (and the plane) will be treated as standard category when they contain fuel.
My motivation is purely 'tankering'. My wife's limit is about 3 hrs (hoping she can extend that to at least 3.5 for what will be a recurring trip). But we have wholesale avgas on our airpark, and I intend to add a premium mogas tank (my a/c engines have always run better/cleaner on mogas). At a $2-$3 per gallon price spread, if I can tanker enough to make a round trip on home field fuel, I can pay off my materials cost pretty quick. I wouldn't have even considered $2k for aux fuel on a -7/-8; the cost/benefit ratio just wouldn't be there for me at that price.