Prices are all over the board, depends on finding the right buyer with gold to spend.
I placed a Castle on Napa on the telestorming day of placing in Tram. Sold it about 6 or 8 months later for 15 mil, after deciding that a Castle was way to big for 1 person.
A few years later I bought a Castle on Ice island (Napa) for 50 mil gold and about 100 mil goods.
About 6 or 8 months ago I bought a second Castle on Napa, for I believe 60 mil gold and about 80 mil goods.
Now the guy that has the Castle where I originally placed back in 2000 or 2001 (I can't remember the year LOL) is trying to sell it for 300 mil Gold. The original Castle fell and was replaced by an idocer on our shard, I believe he sold it for somewhere around 200mil, to the current owner.
(long winded I know)
That all being said I believe that your Castle should be worth 100 to 200 mil. But it's only worth that if you can find someone with that much gold, that's also interested in buying.
If however you just want to sell quick, I'll toss my bid in at 50 mil gold. I don't plat Cats much, but if I had a Castle there that could change. LoL