In Springfield, during the last 3 months, the average Original List Price was $296,488. The average Sold Price was $277,791. I don't think your offer is too low, at all. It is a starting point, and all negotiations have to start somewhere.
In the 1st Quarter of '08, the recorded sales in zipcode 19064 averaged 97.3% of the asking prices, (See page 25 of the referenced report) so I suspect you will end up closer to $260,000, when everything is said and done.
Good Luck. Feel Free to contact me if you need any more help understanding the local market conditions.