You will pay a single supplement - it is not quite as expensive as paying 2 fares. Is there no one you can go with to avoid this? Clue is to do a lot of research so you know what you are doing in each port, and join your roll call over on cruise critic so you get chatting to fellow cruisers before you go - so when you get on board it going to be like meeting old friends and you will not be alone.
There is nothing to fear in cruising alone. I have done so many times and find that I am usually placed at a dining table with other sole travellers so you meet new people on the first night. Ask to be placed at a large table.
The biggest bugbear is the Single Supplements which vary from one cruise line to the other. My experience is that most American cruise lines really milk this one, but NCL is not too bad, and watch out on Teletext, occasionally you will get a deal with no single supplement. If you can be flexible with dates, ships and itineraries you should be able to get a deal, I have been on lots of cruises and not paid a supplement or at worst a nominal amount. I have just booked a cruise down the Nile with a week's stay in luxor and only paid an extra £100 on a 14 night holiday. You will find cruisers a very friendly lot and I am sure you will make lots of new friends. Good luc.