An au pair must never pay for these expenses themselves. All medical expenses must be paid for by the host family, including advance payments.
Many insurers work on the basis of a reimbursement system. In this case, the host family must advance the costs and later submit a declaration to the insurer to have these costs reimbursed. An au pair must never pay an advance. The host family is the policyholder and the au pair is the insured party.
If the host family takes out the cheapest insurance which doesn’t cover 100% of the medical expenses, the host family is still obliged to pay the remainder, which may result in a higher cost to them.
If you are a European citizen and you and your host family have signed (and registered) an official au pair contract, the host family is legally obliged to take out additional insurance for you. Your European Health Insurance Card is not enough!