This varies by field, and it varies very closely in proportion to the employability of a master's degree in that field. A master's degree in biology does not ha

This varies by field, and it varies very closely in proportion to the employability of a master's degree in that field. A master's degree in biology does not have the job prospects of a master's degree in computer science, so biology PIs have far more leverage to keep their poorly-paid grad students (and postdocs) working for them.