I understand people have different budgets for buying things, but when you buy a dog, you must keep in mind, upkeep. What if the bargain puppy has Luxating Patella and needs surgery? At the cost of $2000 per leg, this can be a very expensive proposition. How about Liver Shunt, again another $3000. A back yard breeder doesn't do any health testing, and doesn't know her line, so she has no idea of the genetic faults in the background. With a good breeder, there will be heath guarantees. As far as I'm concerned only the well off can afford to buy a cheap pet. A dog is an impulse decision for many, and there are many expenses the first year, so many people buy a dog, and then can't afford to take it to the emergency room for illnesses that crop up. The first year is expensive, with neutering/ spaying and teeth extraction. It may be best to save your money, and wait until you have a little nest egg.
As far as mutts being healthy, many people just put them to sleep at the first sign of a problem, even for something as minor as a broken leg.
No, morkies will not be any more healthy than Yorkies. Maltese have many genetic problems as well, and finding the best possible breeder, who does testing is your best insurance.