You might try the Perseus Vocabulary Tool
It lets you choose the Greek texts you are interested in (you can select one, a handful, or hundreds), you choose how to sort the words (alphabetically or by frequency) and then you pick whether you want the list to have all the words from those texts, only the top 10%, or whatever. It will output a vocabulary list (definitions and all) which you can get into excel.
A couple of notes for using it:
1. The default output for Greek letters is "latin translitteration," which I think is terrible. Fortunately, you can go to the 'configure display' link at the top of Perseus and change it so that it outputs Greek characters the way you want---probably "unicode(UTF8)", is what you want.
2. To get the output into excel, choose the 'comma delimited' option. Then, when you see the output on the screen, copy and paste the list from your browser into a blank text file in notepad. Then save the file with a .csv extension. Excel will immediately recognize this as file with data separated by commas and will convert it into a regular spreadsheet.
Best of all, it's totally free.