So, for a 83 Spider, the Beck/Arnley part number is that same part number mentioned above... rockauto has it listed for $181.99. That bites...
But maybe with that much info., you've got enough to go cyber-yardin' at the alfa specific sites? Maybe?
BMW Brake master cylinders are showing up for a lot less on rockauto.. I looked at an 83 320i reman'd unit from A1 Cardone and it's listed for $39.99... Not sure if that's the right one you're looking for but I suspect other units from A1 Cardone aill be priced similarly....