Man, I was playing the game the other day, fighting quadav, and I found out that their backplates sell for more than the crawlers silk thread... now you might be thinking that I'm crazy, but Its true, they sell for 1000 in Jeuno.
I have still not found out how much they are in stacks.
Yes, as I level a new job I especially like getting these during a party and later during farming. As one gets older, the gil just isn't enough as there are better drops.
I've killed lots of quadavs, and I'm fairly certain that it's only amethysts that drop the backplates. If ambers can the drop rate is VERY low, far less then amethysts(Who are about half the time with treasure hunt).
Also in regards to the post below, level has no effect on any drops except crystals. I farmed palborough with my thief from around 15 to around 30. I've found better gil farming methods since then, but I recently came back to palborough with my level 40 ranger with thief subbed for old times sake. Amethysts droped the backplates just as well as ever.
When you are a low level and you are trying too get experience this is a good fiend too fight. At a low level the drop rate for backplates is much higher, they are worth 700 gil each on my server.