Igor is exactly right and ...

Igor is exactly right and each system also has a different costs associated with leaks including the cost of the water, damages and the cost of repair, all of which can be reduced by early correction.  For systems with low leakage, low cost of ample water supply or where water leaks surface quickly, a reactive approach may be the best approach.  It may also be that water loss in a system may be attributed to non leak or apparent losses.  The AWWA M36 speaks to the economic cost of water loss and that would also be a good starting point for you.