There are so many different options available to you, it would be difficult to give you an estimate on costs. It would depend on the cruise line, time of year, length, itinerary, type of cabin, etc, etc.
You can also consider instead of doing one long cruise on the same ship, doing a couple of cruises on different ships/cruise lines and spending a few days between cruises in an exotic locations.
As I said, lots of options.
We had a client last year wanting to do a 6-month round the world cruise. We booked them on two 3-month cruises; one did the southern route around the world and then they changed ships and did the northern route around the world. Worked out perfectly for them as they really got to see everywhere they wanted to go.
One thing I always suggest to those booking round-the-world cruises; make a list of all the places you really want to go, then make another list of places you have no interest in seeing. Then give these lists to your agent so they can do the research for you and help find the best fit based on your needs, lifestyle, and budget.
Personally, I'm not a big fan of online booking sites; they're more interested in selling than in providing service.
But this is one of those cruises you definitely need to be working with an agent and let them do all the work for you. And I agree with Trip, it's very important to purchase travel insurance for a cruise line this. Because of the length of time you're gone, anything can happen.