After some digging around at the Michigan organ donor website, www.giftoflifemichigan.org, I found an answer in a seven page online brochure for donor families called When Your Loved One Dies:
"Families are not charged for donation.
There is no cost to the donor family. Donation is a gift. The
family of the donor is responsible for hospital charges not
involved with donation and for funeral arrangements. The
family should never be charged for anything related to
donation. If you should receive a bill from a hospital or a
funeral home that you believe includes questionable costs,
please call Gift of Life Michigan at (800) 482-4881 and ask to
speak with someone in our finance department."
Excerpt from When Your Loved One Dies, Page 3.
It appears that Gift of Life Michigan wishes to assure you that there will be no costs charged to you or your loved one's estate for the donation, and that they will provide assistance with any questionable billing issues.
If you'd like to register to be an organ donor, visit the Gift of Life Michigan website. If you know of a family who was charged for donating their loved ones organs and/or tissues, I'd like to hear from you!