I agree with the principle that you should charge a day rate for a day's work. However, I don't agree with the author in this case. For the author, this is an

I agree with the principle that you should charge a day rate for a day's work. However, I don't agree with the author in this case. For the author, this is an unusually genuine opportunity to see what the work is actually going to be like. If the author feels that compensation for this time is necessary, they ought to confirm the salary would be at a level to cover the time-cost and risk of interviewing. Asking for money up-front is (unfortunately) sending a signal that you need to be compensated early because you expect not to be employed as a result of your work.