I am signed up for my 10th ABD as a solo traveler this summer (Iceland)! It's a fabulous way to travel as a solo! But yes, there is a 40% single supplement that you have to pay on top of the regular per person price.
If you're looking at the River Cruises, the single supplement is, unfortunately, 100%. You pay the same amount as 2 people in a cabin. It really sucks. The only way I was able to afford it was because they had a promotion for a 50% single supplement when they introduced them.
Let me know if you have any questions about doing ABD as a solo. I'd be happy to answer!
Have done five solo ABDs, and doing Six and seven this summer. You are always upcharged, but that's with any company. I've enjoyed every trip immensely. If you can afford it, do it.