Financial return on rare breed isnt the best, if it worked then it would be a commercial breed and not rare.
I do something similar, with about 15 a year. If you take out the cost of the housing and just take into account hatching eggs, feed then yes you can make about 25% return on it. If you put housing into this, your time to kill/dress them, then your going to have a hard time making it pay.
Money wise you looking at selling about £3.50 a lb, thats the price in the North of England. So a 14lb rare breed, say a Bourbon Red, then it will be a sale price of £49. However if your putting them into the meat markets, where people dont really care, especially where I am, you wont get that price. I find it best to advertise locally, farm shops etc, people who are a little more into where their christmas lunch comes from. They also tend to be the people who dont want a 22lb turkey, will be happy with 14lb and dont want to be eating it until New Year.
I know its not the most helpful post, but all I would say is if you have a place to keep them and can buy your feed in bulk then yes you could make it pay. Just.