The AdWords Keyword Planner tool will show you accurate data for search volume, average click costs, and competition levels, however cost estimates are just that, an estimate based on the average performance of the average advertiser. If you can perform like an average advertiser then it should be pretty close to what you can expect, but that’s a big if. The CPC will very quite a lot for each advertiser based on the individual advertiser’s Quality Score.
Since an advertiser that has earned a superior QS may be paying only a fraction of what the advertiser with an average QS is paying, estimates based on averages isn’t going to be very close to actual results for most advertisers. You have to adjust what you see in the estimates to reflect your own QS and also understand the general makeup of all of your top competitors before the number can be used for a reasonably close estimate.
In addition to Quality Scores varying you actual costs, ad positions and location targeting are also major variables that will effect your actual cost. Without all of those variables taken into account, estimates based on averages are pretty much useless, except for comparing one keyword against another to see which are relatively higher, or lower, in cost.