Market value vs. asking price, is it a good deal?

[Jordan Hanksion]

Hey guys, 

I have began searching for deals in my market (Madison, WI) but I'm still struggling some to understand what makes a "good deal". Now I understand that I should be looking for an investment property with a final purchase price 80% of market value or better, however I am having difficulty understanding what determines "market value". Is market value simply what the tax assessor has listed that the property is worth or is there more to this calculation that I am unaware of? Or in the same breath, am I just wrong in my figuring all together?