In the United States, many hospitals as well as health insurance companies charge universities to access patient data.
How much did MIMIC have to pay hospitals to get access to "their" data?
While hospitals charging for data access is a disappointing trend, I don't think MIMIC paid for access in the sense implied by the question. The research and development of MIMIC is the product of collaboration among MIT, Philips Healthcare, and Beth Israel Deaconess Medical Center sponsored by National Institutes of Health grants: NIH-R01-EB017205, NIH-R01-EB001659, and NIH-R01-GM104987.