The added cost of having an additional person to provide room and food for way exceeds that €300/month. Especially, when taking into consideration that you might have to extend/renovate the house to lodge another person. Adding an extra bedroom and possibly bathroom is not cheap.

Even if you assume the cost of lodging was 1000€ (which it isn't) then the au-pair would still be significantly underpaid.

A normal full time employee costs at least 2000€ a month (salary, tax, pension plan, health insurance, etc). If you are paying less than that you are definietly exploiting them.