Just had a look on a couple of servers I've had characters on...
These numbers are from when Void Ark was released til now...
On Ridill, there's been about 4-5 coffin lids... selling between 3 million and 4 million.
On Midgardsormr, no coffin lids EVER sold on the MB... currently one coffin being sold for 30 million.
On Siren... there have been none... No Coffin Lids OR Coffins sold... EVER.
Meanwhile, back on Balmung... we've had 8 coffin lids sold, one up now was originally selling for 35 million, then down to 25 million... now back to 35 million. We've only had 2 Coffins sell on the MB (none up currently) for 15 million each.
There is something wrong here, where a known wanted drop, has such a terrible drop rate... Even the damn Spectre/Premium Spectre/Armory packs for Mass Effect 3's MP (which were notorious for RNG for drops... trust me, I know for a fact), have a drop rate for Ultra Rare/N7 drops of nearly 80% compared to this... and it was confirmed to me by Chris Priestly during the first N7 Day celebration that the drop rate for Ultra Rare/N7 drops was a floating percentage between 1% and 10%, EACH pack.
I reference the ME3 MP, as comparison here... Even drop rates for items in FFXI have a far greater percentage of dropping than this one piece.