If you have already kept dogs, you would have realised that if you keep them well and feed them well, there is hardly any money that you can make. Its more of an hobby rather than a business. Also compare the time and effort put in and the rewards. They are like children and you get stuck to the house, unless you have good domestic help which is hard to get.
Breeds may be popular, but at the same time you will have so many people breeding those dogs that the cost goes down, demand and supply, hence the pressure to sell at a cheaper rate, inferior quality, etc. Its not that you want to sell at a high rate or where u can make a decent amount end of the day, but the price prospective owners are willing to pay and the choices available, as new owners generally dont know much and just want a cheap puppy.
Are you willing to hold on to pups if you dont get the price you want, are you willing to keep them as pets in the house if they dont get sold to good homes, have you calculated the cost involved in keeping a dog for the rest of its life and the amount you may think you can get during its breeding years. Give it a thorough thought and then make what ever decision you have to make.
Decide wisely and all the best!!