There are some sailings that have "single supplements" that are less. That means you may be able to get another room for 125%, 150% of the double fare.
VTG you can click on singles discounts on the left hand side of the home page. I'v found as few as ONE carnival cruise out of what 23 ships and all the cruises for Carnival. ONE.
I check regularly since Im cruising solo these days and the Carnival cruises with less than 200% are few and far between. Scarce enough that to me for Carnival lately, they just do not want solo cruisers I am convinced. No other cruiseline has so few less than 200% cruises.
The ones that I find for all cruiselines are almost always out of FL.