Mack maintenance cost (94 e7)

All makes have parts that are proprietary and they tend to be expensive. Need a 1 foot square plastic cover for the fuse - breaker box in front of the firewall on a 2006 7600 IHC. They want over $200 for that. Most everything has become expensive.
Had a Mack a lot newer than 1994 with 3 exhaust manifold studs broken off. Called to see if they had 12 manifold studs and nuts with 6 gaskets. Four turbo studs and nuts with a few more gaskets. Signed a blank check and set a runner 45 minutes away. They came back 900 and some dollars latter. That stuff can be found on line for much less. I could go on and on all brands.
Macks used to be a little different. If they had a Mack engine, transmission, rear ends and suspension they had a gold bull dog on the hood. If someone special ordered one with a Fuller transmission or Cat engine they had a silver bulldog on the hood.
If you have a Mack with Rockwell diffs that is common stuff many other companies are running. Prices are competitive but if you have Mack double reduction rears they will cost you more.
When trucks get to be 30 years old can you still buy cab parts regardless of what they cost?