I can only find one in trades and there's no asking price for it!

well when it appear it was 100m~50m and people were buying them quite fast. Hardcore collectors, most got it already. I have buy 2, mine cost me 250 csc and other one was like 400 or 500csc. There was another one that sold for 100m.

I will just throw it in shop for 100m and maybe lower the price like 10m every week if no other pops up. If another pops up, just go below that one.

I disagree. Right now would likely be the worst time to price it in shops. It's old enough (Came out mid-October) that the "hardcore collectors" already have it, plus the free shop is giving them out, so the price is bound to deflate. Your best bet would be to try to sell it quick (Maybe trading it for something more stable in price) before the price deflates too much, or to hold until after the giveaway and/or it stops RS'ing in the BBH.