I don't know if there's an over all rating number for the armor your wearing. as for your example it is comparing the weapon damage of the one you have currently equipped to the one you're picking up, your using a 22 damage sword and find a spear that's 18 then: 22 -->18. or in reverse holding a 18 spear picking up a 22 sword: 18 --> 22.
i've never seen a price tag at wal mart that was a whole dollar amount, 399.99 or 399.95 i can see but 399.00? that's strange ... and by strange i mean fake. that said i could see 300 or 350 (bundle) being fair price i'd jump in early for.