Ducale/Ducalis Roger II Palermo Sicily

Today I got received two items I have been hanging out for the first is the reference Medieval coinage vol 14. Getting that book means II can read up on the second item that arrived, the Ducalis. I had been hunting this coin off and on for about three years. It hasn't been high on my priorities so I probably missed out on a few opportunities. This is one coin( on my hunt list) I don't know that much about. The reason I wanted an example was that I understand this coin is the first "Ducat". I have some reading to do to find out about it but would welcome comments here. I will comment on the buying process.
In getting this coin I really was going blind, I had(have) no idea of its rarity and market value. I just know when I did look it was hard to find. I posted on a dealers page on Facebook a few weeks ago asking for indications of its value and the immediate responses were that it would cost in the vicinity of $1000, or a good part thereof. A week or so later another member of that group offered me an example for $1200.00. Before accepting the offer I did one further search, finally thinking like a hunter, and found an Auction house was selling one in San Marino. I don't buy often from San Marino as its a small state whose status in the EU customs Union can be problematic. I believe I had to pay duty on a previous purchase( but my memory could be wrong. I also, had another collector point out one had sold earlier this year for €600. I took a gamble and turned the $1200 offer down and bid on the one in San Marino. With postage and buyers premium I got this example for €380.00. I don't think the $1200.00 offer was off I just think there are so few sales of this coin that there is no firm market value. The other issue had was having in my search was actually what the coin was called. Some described it as a billon scyphate of Roger II. that's what I had usually searched for. I found this one searching for "Ducale" the Italian name for the coin. I am pleased with the example of have got although I think the $1200, which looked to be in the same condition when I compared photographs, might have been the better coin. I say that as although it looked good in the photographs when I photographed it I discovered there is a crack in the coin. the crack is hard to see by naked eye. You can see the crack more clearly in the hi-resolution image which is here( warning you can only view it by downloading the 168mb Tif file).
https://drive.google.com/open?id=1P...S9fvAuzLOtIG

Even with the crack, I am happy with the coin. It is from Palermo and struck between 1140 and 1154. It was issued by Roger II. I have attached a much lower resolution image to this post( you can spot the crack in the reduced image)