click on them you'll have the explanation.
the first is about the number of commits, the second is about the amount of commits per month. it doesn't say if you commit 500 patches of one line or one patch of 500 lines. the second will be seen as less activity.
the "project cost" is only for the code written and it's based on cocomo (iirc, if it's not that it's another model with the same goal) which gives you the amount of month/person needed to develop a software starting from the line count, usually, and from there you can determine the cost.
you can find more informations about it in any software engineering book or wikipedia. the same ohloh site gives a nice overview too