It is always OK to ask for closing costs, no matter who the seller is. But there is a difference between asking and expecting an affirmative response. Short sales are very difficult to negotiate closing costs because, since the seller does not have enough money to pay their mortgage, you can rest assured they don't have enough money to pay your closing costs. This means that the only one who would be able to contribute to the closings costs for you would be the short-pay bank. Their objective is to minimize the loss on the property, and paying for your closing costs is contrary to that objective. With a foreclosure it is typically not as difficult to get help with closing costs, but more often than not there is not closing cost credit on a foreclosure sale. Regular people selling their house with sufficient equity will typically be least resistant to closing cost credits.
The bottom line is, if you need help with closing costs you should ask, no matter who the seller is. Of course, the seller is free to say no - but, nothing ventured, nothing gained. If you need the help and the seller can't or won't help, then don't buy that house. Chances are, there will be another just as good or better.
The answer to your questions about how much your closing costs will be depend upon where the house is (closing costs vary from state to state; and even from county to county, city to city), and your loan program. Closing costs can more or less be divided into two categories - transactional costs and loan costs. Your local Realtor should be able to estimate transaction costs for you (title & escrow fees, transfer taxes, etc.), and your mortgage lender should be able to estimate loan costs for you (points, fees, mortgage insurance, impounds, etc.).
