How do you determine the true current value of an apartment??

Hi all, I'm about to purchase my first apartment and I could use some advice. I got an accepted offer on a place that the broker believed was a really good deal (supposedly an extra good deal since I'm paying in all cash). Since then I managed to get information on past sales in the building. In 2006, a higher apartment on the same line and completely renovated sold for $42,500 less. In 2005, a similar apt in the building sold for $131,000 less. Given todays market, should these differences raise flags? How do I determine what the value of the apartment is? Any information would be appreciated. Thanks, dk370