I cruise solo. The standard single supp rate for Carnival is 150% of the double occupancy rate for a Cat. 1A cabin. If you go any higher than Cat. 1A, then you are charged 200% of the double occupancy rate. I have never booked anything but Cat 1A. when going solo. I just don't care enough about my cabin, its location or having a window or balcony to pay that much money when I can get on the ship for so much less.
I have seen the single supp rate as low as the double occupancy rate plus $15
It all depends on how well Cat 1A's are selling, how close it is to the date of the cruise, time of the year.
For my eight day cruise on the Liberty next week, I paid $125 over the double occupancy rate for a Cat. 1A approximately 112% of the dble occup rate at the time).
You just have to keep looking at the prices and have some level of flexibility.
If you will not be happy in a Cat. 1A, then expect to pay 200% of the double occupancy rate.