First time Doxa buyer?

I’m new to the watch world and have spent a lot of time just learning what I like. One of the first watches that struck my eye was the Doxa Sub 300 in the classic orange dial. I’m looking to buy. What’s a fair price, a good price and too high price I will expect to pay? My expertise is in firearms and know the market on most guns, I wish I could figure out the watch market as well.
Thanks for any advice, comments and constructive criticism.