Oppressor Weapons depend on market value of the War Supplies. It's been steadily declining for since it spiked up to >100k upon release of HoM calculator. Most recently, I have noticed a few sellers in game offering Oppressor weapons for ~70k.
Destroyer weapons' PC is based on the going rate of the crafting materials to make them ... iron, granite, diamonds and onyx. Last I checked on Nov. 30, the cost to craft a Destroyer Weapon was 50-64k.
For what it's worth, both Oppressor's and Destroyers are things we (try to) track on the sticky thread here in PC,
Greens & Other Unique Items (all campaigns) Price Guide (That list is only as good as the reports of prices we get, so please do feel free to post the prices you buy and sell such things for!)
Cheers & GL!
EDIT - Oh, and the demand for Destroyer things spiked with the release of the calculator. Thus, the demand for the crafting materials spiked, too, and their prices at the NPC material traders went up. Before the calculator's release, the value of crafting materials (especially the diamond and onyx) was a lot lower. So back then it was rather routine to see Destroyer things for 20-30k or less. This is no longer the case, but that's probably why your friend was thinking 30k.