There is no "standard" amount. I suggest different amounts, depending upon the type of sale it is.
If the property is a short sale, I would recommend no more than $500 for an earnest money deposit, since the money may be tied up for a long time, while you wait for an approval of the sale by the Seller's lender. For a foreclosure, perhaps $500-$1,000.
If the sale is a "normal" sale, then I usually suggest 5% of the amount offered. The reason for this is that a substantial deposit is a means of indicating the seriousness of your intent to the Seller.
Good luck with your purchase in Norristown. I lived in the area for 18 years, and have fond memories of it.
Maggie Hawk, REALTOR
Watson Realty Corp.