If a house is in the "pre-foreclosure" stage, that doesn't necessarily mean it is for sale. And be careful: the numbers some real estate sites put on those homes don't represent prices, they represent judgment amounts.
If the owner does decide to sell the home, and it is a short sale, you may get a small discount off market value. But depending on the market you are looking at, you should still expect to pay at least 90% or more of the full market value. Same situation if the home eventually is foreclosed on - usually it then becomes a bank owned ("REO"), but again the discount isn't extreme. And it is easier to get a better price if you are a cash buyer (and there are lots of cash buyers out there).
Bottom line, if you are interested in purchasing a home, hire a good agent who has some short sale training/certification and he/she will help you look at all avenues. But be prepared that there are no hidden bargains out there - no matter what path you take you will be paying somewhere near market value for a home. Good luck.