Those exhausts are modern examples (still made by Honda I understand) you can tell by the notice! David Silver sells them for £75 each + VAT + shipping (that's about $750US at current rates)
Here's a comparison of CB500/550 exhaust prices from two different online sites, per their Fiche...
1973 CB500K2
RideNow
04 MUFFLER 1, EX. 18300-374-707 001 $174.07
07 MUFFLER 2, EX. 18350-374-707 001 $172.42
08 MUFFLER 3, EX. 18400-374-707 001 $137.35
11 MUFFLER 4, EX. 18450-374-707 001 $128.32
Total $612.16
MR Cycles
04 MUFFLER 1, EX. 18300-374-707 001 $162.46
07 MUFFLER 2, EX. 18350-374-707 001 $160.93
08 MUFFLER 3, EX. 18400-374-707 001 $128.19
11 MUFFLER 4, EX. 18450-374-707 001 $126.42
Total $578.00
I couldn't find a Fiche listing for a 350 Four in 1973, but there is a 74 listing -
1974 CB350F1
04 MUFFLER 1, EX. 18300-333-617 001 $195.29
06 MUFFLER 2, EX. 18400-333-617 001 $229.43
10 MUFFLER 3, EX. 18500-333-610 001 $193.01
11 MUFFLER 4, EX. 18600-333-610 001 $201.08
Total $818.81
Quite interesting...but for the life of me, I can't figure out why the individual pipes [1 through 4] don't all cost the same...