The fee should be clearly established with the client before the assignment begins. There is little point continuing with the assignment if either you or your client is unhappy about the fee.
If the valuer is working in a large organization then he/she should make sure that the fee structure has been determined according to company policy. This policy should also include how and when the fee is to be paid. Often it is payable when the client picks the valuation up or having the client make a part payment beforehand.