White and Purple minis people usually give away for free, though you might get 1-3k each for them.
Gold minis maybe a few ecto each and green 5-10e, still a hard sell though!
You can use this site to search trade chat history in Kama - search for "unded" + mini to check the recent prices.
I don't have the link to hand but there's also a GW discord with a trade channel where it's worth just posting your WTS mini list in addition to GW Legacy