If the property is more than what you are willing to pay, you may have a problem. Foreclosed homes are priced at or slighty below market value so that they sell quickly. Of course, this is not always the case, but I agree with Alma and Diane as knowing what part of the foreclosure process the property is in will help you determine how you should proceed. Please note: Most homes will sell for what the market will bare at that specific given time; if there are more buyers in the market for the same property, prices will go up.
Before making your offer on the property, visit the property with a Realtor of your choice so that your agent can research comparable sales and consider the condition of the property as well.
Depending on condition, the property may not qualify for a conventional loan or FHA loan. You may have to buy through FHA203k (Loan + Home Inprovement Costs).
I know your question was about the "percentage" the banks discount homes. The truth is, some properties are reflective of their condition and not truly discounted. Just my thoughts. I hope it helps.