Good question.

First, 5 SNG's is completely useless for real analysis. This type of tournament has very large variance. Once you get a few hundred samples the results will be more accurate, but still not that meaningful. Players who play this type of tournament online usually play many, many tables at once to lessen the variance of the session. I played these for a while, but I got so bored I had to quit.

As for your calculation, you're trying to analyze the ROI - that is the ratio of winnings to entry fees payed. So you are dealing with a proportion, and the answer can be calculated using the appropriate formula from beginning statistics and using distribution tables.