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...   
 
  
 