You can see statistics for all your lines in the bottom right of the Soulver window.
The statistics will be visible only while there are answers in the answer column.
Soulver supports 4 different statistics:
Total (the sum total of all your lines)
Average (the mean of your lines)
Variance
Standard Deviation
Click on it, and choose your desired statistic from the popup menu.
Click on the statistic and choose "Copy Selected Statistic" from the popup menu.
Tips
To see statistics for a specific set of lines, just highlight the lines in the text editor that you want to be calculated into the statistic. You can also select answers in the answer column.