That would be called therapy sessions rather than singing lessons. Fees for healers can hardly be standardized; often the payment is the most effective part of the procedure; this is called the placebo effect.

As for lessons, my advice would be the following: for the first two years never hire a star singer, even if money is no object. Rather insist on someone who thoroughly understands the basics and can explain them to you. Such teachers do have their price, but nowhere near the amount in discussion here.

The same applies to teachers of instruments.