"You will be asked to make a pre-service deposit of at least $750.00USD. If prescheduled services are estimated to exceed this amount, an additional deposit may be requested. The pre-service deposit can be made in the following ways: major credit card, wire transfer, check (we accept checks from any US bank in US dollars) or by cash. Please know your total and daily credit card limits."

Mayo Clinic in Scottsdale AZ, November 2017.

edit: I'm a bit off topic, that was for an appointment, not ER.