You have two quality modifiers there. That only really occurs for building designer, I think? In traction benches parts are discarded. But who knows.
A few examples of values in ascending order in my save:
wild boar leather mid-size hand-held wind i. w. wild boar leather/giant hedgehog bone/ash/silver, decorated with rings of reindeer bone &-wombat bone- = 210
earthenware mid-size hand-held silk-stringed i. w. earthenware/earthenware/Enslaved Doom silk/sea serpent bone = 270
Well-crafted green glass huge stationary i. w. metal-stringed green glass/iron/blue peafowl bone = 330
Silver mid-size hand-held metal bowl i. w. silver, no quality: 500
Well-crafted crystal glass large stationary silk-stringed i. w. crystal glass/brown recluse spider silk = 1110
Well-crafted large stationary silk-stringed i. w. wood/wood/wood/clear blue cloth = 3130
Fourth the bowl is the easiest: total 500, silver is 10, therefore item value is 50.
First, without decorations would be 180. Everything but silver has modvalue 1, therefore the multipliers might be something like 10/10/10/15.
Fifth is also relatively easy with 2 parts. Crystal glass is 10, brown recluse silk 1, well-crafted is x2. 555 could be divided into 50*10 + 55*1. For comparision, a green glass no quality body part in same caravan is 20.
I can already conclude the instrument values are proc-genned, of course. This means that if one is to experiment with trading instruments, they must craft samples first.
E: Raw dump of names/raw item value claims all instruments have 50 value, which doesn't seem to work for multi-part instruments.
E2: Interesting. Disregard above calculations for well-crafted stuff at least - Setting the 2nd to well-crafted gave it 420 item value - when doubling would have been 540. Seems the instrument parts are kept as decorations.
This comes to main part value: 150.
earthenware part 1: 30
earthenware part 2: 30
Dom silk part: 10
Sea serpent part: 50
The total does check out to 270. Earthenware has 3 modvalue and sea serpent 5, so the parts are 10 each and main body is 50.
Thus instrument value for single part instruments is same as wheelbarrow/minecart, and for multi-part instruments each part is added on as a decoration, with the quality (but not the improvements, I guess?) of each part kept.
Using this, the fifth would be 2*10*50+10*10+10*1, which is indeed 1110.