Mehudar Mezuzah

A nice Mehudar Mezuzah can take a qualified and Certified Sofer anywhere between four and five hours to write. Before he writes he makes sure to immerse in a Mikvah. When he writes he makes sure to say the letters and words to infuse them with the Kedusha of the Torah. 
He checks each line over before writing the next, He cares about the end product. He takes pride in his work, in a sense like an artist takes pride in his painting, but even more so the mere thought of a letter being written incorrectly horrifies him.
 A Sofer who is a Yirei Shomaim wants his Mezuzos to draw down Hashem's Kedusha/Holiness in to this world, and he is honored to be able to be the vessel in which it comes about. He is not primarily writing for financial benefit. 
As you can imagine because so much time and effort is vested into  nice Mehudar Mezuzah they can easily cost one hundred and fifty dollars, while some can reach as far as two hundred and fifty. 

The more Mehudar it is the more it will cost.
Mehudar means two things:
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A) Aesthetic beauty, the Torah says “ze keli vi anveihu” which translates to “this is my G-D and I will glorify him” we learn from here that it is proper to adorn the mitzvos, that our observance of the mitzvos be a beautiful as possible. In our terms here it refers to a beautiful Ksav.

B) More importantly it is referring to the Halachik aspect, to fulfill the Mitzvah on the strictest level. A level that every Halachik authority would agree  is Lechatchila and that it is the most kosher it could be.

But remember just because a Mezuzah is not Mehudar does not mean it is not Kosher. There are several factors that make a Mezuzah Mehudar. The more they have the more Mehudar they are:

1) Beautiful Ksav. Simply it means that the letters are beautiful, their length and height are symmetrical, and the Sofer followed closely the intended Ksav and did not veer off.

2) That the letters are formed properly and that there never was any doubt over the form of any of the letters.

3) None of the letters are touching from the time of the writing (there are opinions that say that any letters touching from the time of the writing even if they don’t loose their form or form another letter is invalid. Although the Halacha is not like them for Mehudar we want to encompass all Halachic authorities).

4) No questionable spacing, the words and letters need to be spaced properly and evenly as to assure which letters belong to which word.

5) The letters have the crowns written on the letters as the letters are formed. In many  cheaper Mezuzos the words are first written, and then later someone else adds the crowns afterwards.(there are opinions that say they need to be formed as part of the letter.)

6) All the letters that need, have additional crowns as specified according to Kabbalah.

7) According to Halacha the letter Lamed has two crowns/tagim. Kabbalah explains that the tag on the right side corresponds to kindness, and the one on the left, to severity. Consequently, it is of utmost importance that the right tag be taller than the left one, ultimately drawing down kindness from Above.

8) The names G-D on the back of the Mezuzah are in the correct place corresponding to the names of G-D on the front of the Mezuzah.