When you indicate only one passenger, it usually doubles your cruise fare (you're paying for the first AND second passenger). But Royal's current sale is for 75% off the second passenger. My starting fare for the 9-day Constellation cruise was $3,480. But I had over $2,300 in discounts. I booked through my TA, so she makes sure I benefit from Florida resident discounts, etc.
I booked my 4-night Celebrity Reflection cruise on November 16 last year - when I priced it for a solo online, it was only around $687. By the time I chose my room and the additional taxes and fees were added, it came to about $820. They also had the classic beverage program, premium Wi-Fi, and Coco Beach Club passes on sale, so I took advantage of those lower prices, as well.