The department shall investigate and determine the assets of the estates of each resident of a residential habilitation center and the ability of each such estate to pay all, or any portion of, the average monthly charge for care, support and treatment at a residential habilitation center as determined by the procedure set forth in RCW
43.20B.420: PROVIDED, That the sum as set forth in RCW
71A.20.100 shall be retained by the estate of the resident at all times for such personal needs as may arise: PROVIDED FURTHER, That where any person other than a resident or the guardian of the resident's estate deposits funds so that the depositor and a resident become joint tenants with the right of survivorship, such funds shall not be considered part of the resident's estate so long as the resident is not the sole survivor among such joint tenants.