Most online agencies will not have experience with the intricacies of World Cruises. I would find an agent that regularly books world cruises.
Among the mainstream lines, Cunard, Princess and HAL have world cruises.
Among the luxury lines, Regent and Crystal have world cruises.
There are probably additional lines that I am missing.
Prices vary greatly. You could probably go on the entire 162 day Regent Seven Seas Voyager this year for around $70K - $80K per person but everything is included. If you sailed Princess, your fare would probably be under $30K per person but tips, tours, alcohol, etc add up.
Bottom Line: You have a lot of research to do.