who pays the deponent's costs incurred for attending a deposition?
If the deponent is a party, he’s on his own for all expenses.
If the deponent is a non-party, a witness fee (as per your court’s rules) should accompany the subpoena. The witness can also ask for travel expenses (tolls, parking, cab fare, etc.). These are the only payments I know of that a non-party witness is ENTITLED to, but your state or local rules may differ.