The price calculations in Divine Divinity is very different from Diablo. Item's actual bonuses make a much smaller contribution to price compared to base price of prefix and suffix.
In Your particular example,
Fiend's is a cheap prefix (with no suffix at all) while
of Matter is a very expensive suffix (with medium-priced prefix too). So it is not a bug, just a strange pricing mechanics.
P.S. By the way, in real life this also happen sometimes. For example, Apple gadgets can be 8 times expensive than cheap analogs with same functionality.
