Firebrand is rare, yes, but I see no reason it should be more rare than any other WiK green dropped by a mob. It's elevated price is largely due to it being a unique skin (not obtainable anywhere else), with desirable stats on a desirable attribute. Peacekeeper Firebrands can also be found in plentiful numbers on numerous maps with zero effort.
The Thunder, on the other hand, is a very basic skin with typical stats on a very unloved weapon type. It's value comes solely because Jormungand himself is an absolute nightmare to try and find. If it were anything fancier I'm sure it'd be up there in value as well.