What is the reasonable amount for a facility to refund patient care monthly charges upon death?

When a patient dies on the 11th of the month, and the facility issues a refund based on much higher than actual charges such as $175 when the actual charges per day for the last 2 years was $105 (based on a monthly charge of $3200 per month) and the "incontinence fee" of $400 per month is calculated at $15/day instead of 13/day. I calculate they owe me $2188.14 rather than the $810 they have offered. What should I do?