Standardizing monthly sales figures in excel

Hi everyone, this is my first post.

In excel, I have approximately 723 rows of sales figures, broken down by month. I have an average monthly purchase (in Dec. 2014) of $24,500.47 and standard deviation of $76,232.24. The problem is, many of the rows have a $0 purchase (that member didnt purchase that month). As a result, A TON of my standardized values seem to have a min of -.32139512, which indicates a $0 purchase.

How should I go about adjusting my data, so that when I go to graph a PDF of my standardized values, it resembles a bell curve more closely?