Hello! I haven’t come in for a long time ...
An urgent topic for practitioners, but has been discussed more than once. We work according to the old scheme: consumables + fee for a successfully completed task. Each time, we first figure out how we will do the work, evaluate (calculate) the expenditure part, and stipulate the fee for the work. Upon completion of the assignment, the results are transmitted to the client after full payment. The main thing is not to mislead the client regarding their capabilities (as some write on the sites “we solve EVERYTHING!” Or “there are no impossible tasks”), as well as regarding the possibility and timing of the task. If you are not sure that the work is feasible - just do not undertake!
With respect to the detectives, Andrei (Simferopol).