Newbie Question About Appraisal Values

[Tristan Wheelock]

Hi everyone I'm a newbie investor just getting my feet wet analyzing deals.

I'll often look up the properties that I'm interested in on the county appraisers website. The variance between the asking price for the homes and the appraised value is all over the map. Sometimes they're within $20k or $30k of each other and other times that asking price will be more than double the appraised value. 

I'm wondering if there's some sort of rule of thumb that dictates how one uses appraisal value to determine price or if there's some resource I can read to learn how appraisal value affects a property. Or if I'm just approaching this wrong. 

Thanks so much!