This is a very specific question that is going to need research far exceeding a look at fair market value for the area. You see, to ask whether or not a certain price is realistic you need to not only concern yourself with the value of the area but also the characteristics of the house, and more importantly of all, the characteristics of the seller.
There is a process I follow in situations like this that has performed very well in the past. Multiple venues need to be explored and certain questions need to be asked. Shortcut the research and you may be unwittingly burning a bridge with a seller when you could have struck a deal or possibly paying more than the seller was hoping for.
The easy answer: check the values in the area and come to a consensus on fair offer price.
The real answer: Do the work, check the specific situation of the property and the owner and explore the venues that many agents are unaware of checking. Take advantage of leverage when it is there and keep a very open mind when you are negotiating terms and price. If you are versatile enough you would be surprised at what you can get.
Luxury properties tend to negotiate with a very different set of rules and expectations than most other properties.
To more specifically answer your question, Is $600k unrealistic? I don't know yet. The research would have to be done and it is a lot of work to do for someone I don't yet know. If you would like to have an actual, serious discussion about it, (760) 464-1685
I wish you the best either way,
Josh Herbel, Realtor