What is the pay per day?
Pay Rate of World Bank's Short Term Consultant

My advisor gave me the following formula when I first consulted for the Bank:
Take your salary (or stipend) plus the value of your benefits (health insurance, 403(b) contributions, etc.) Divide that by 50 for the number of weeks one typically works in a year. Divide that further by five for the number of days one typically works in a week. That's your day rate with the bank.

You can make $300/day with a M.S. and around $400$450 with a Ph.D. They have a pay schedule they start with, and then they look at what others at your ed/experience level make in your office, and you negotiate from there. I've seen early career people in the field make from $200 to $400 per day. Keep in mind, however, that if you are a U.S. citizen, you have to pay SelfEmployment taxes on this, which are essentially the taxes you pay on income + what employers generally pay.