How much does a private investagator cost in a child custody case?

I know it varies for each case but I am try to find out if this is something I can afford. I need a investigation to determine if the otherparent is treating the children properly, ensuring their safety and not exposing them to improper environments or situations. I need to obtain evidence of the children's living conditions, treatment, and to identify third party persons who are given access to the children.