How much to charge per yard in a lawn mowing buisiness?

That depends on a number of factors.

Is the grass collected or mulched? Collecting costs more.

How big is the lawn?

Does it include trimming and edging?

How much trimming and edging?

Is the lawn flat and free of sticks, obstacles and trees? Or are there many things that need to be mown around?

How much time will it take you to do the yard?