Hmmmmmmmm, I don't find this expensive at all.
If you by a complete set of calibre and discs and brake shoes for any other car's you'll find out that in some cases it is even more expensive.
Then you got to have in mind that this is not standard Fiat 500 parts but special handmade racing parts.
Many Tuning and racing parts for Fiat 500, 600 and 850 are at the price level as formel 1 parts because they not are in production any longer and are an especially handmade part.
Anyway... The 850 disks are mostly used on a Fiat 600, because they fit directly on the 2 bolts. I never seen 850 brake parts on a Fiat 500.
Normally on the Fiat 500 one has to use the spindle from a Fiat 126 with 3 bolts instead of only 2 on original Fiat 500 for mounting the adapter plate for the brake calibre.
The disk and the calibre are normally taken from Fiat Uno. but one has to resize the disk seize for fitting inside the 12" wheels.
It's quite complicated to make this kit from scrap parts, and I do think that the time one has to spend on doing so is more worth than 220 ?