I know this isn't exactly what your asked, but I thought I would share my source and experience with you on another angle.
I shave my head about every other day, and I buy my razors from
http://www.greatrazors.com
Their "name brand" shaving cartridges are Cryogenically Tempered. On their website they describe it as Cryo process that freezes the razors in the package down to -300 F. Supposedly the process improves the durability of the metal.
I get the Gillette Fusion ProGlide 8 Cartridges at about $29.85, which is not much different than buying them in the store.
I have used one cartridge up to 30 times on my head. I could have gone more, but it was starting to tug a little.
Before I used to used the stock razors a few times and then change them out. Now i easily go 10 - 15 times and it's hardly noticeable.
Like I said, I could push them out longer, but sometimes it just feels good to get a fresh shave with a brand new razor.
The fact that the price is so close to store prices, it's worth it for you guys to experiment and see if you get positive results.