Yes, milky spore does work, but it takes a little time for it to become established in the soil. You won't see results for at least a season.
Obviously cost is a consideration, but to give you customers more immediate results you can combine the milky spore inoculations with a heterohabditis b. spray. The benneficial nematodes will go to work right away on the grubs and will help with a variety of other root eating soil dwellers as well.
I recomend doing a little research on the best time of year for application; going from memory I believe the grubs are heading into hibernation and when they begin to move closer to the soil surface in Spring they won't be feeding but instead will be preparing to emerge as adults. I don't think the nematodes or milky spore are effective at this life stage. I think the best time to apply is while the offspring are young during the Summer months.
You can also use this infestation as a selling tool for your organic lawn care service; with an explanation of how organically maintained lawns don't have troubles with insects or pest. Explain that if the food web were in place the harmful insects would be held in check by their natural predators, but due to the use of insecticides and other circumstances the benneficial insect populations have declined leaving holes in the food web. These holes in the food web are what allow the pests to gain a foothold in the landscape leading to visual damage. It's always something that we don't see that brings about the demise of our plants that we can see. Preventative measures means starting an organic program before we see the damage occur. While the organic programs may seem costly; it's easy to see how those cost can be offset by damage that didn't need to happen.
And incidentally, if any of your customers have those Japanese beetle bag traps, get rid of them. They attract more beetles than they kill. Tell them to give them to their neighbors. ; )