45€ doesn't sound like much. For a complete apartment cleanup, including bed linen and etc, you need at least three or four hours, so that's around 11-15€/hour. Seems quite reasonable for Austria.
Yeah for Austria it doesn't seem like a bad price, it's just weird that the price is nearly the same as the accomodation fee. And as a Brit the idea of any kind of service fee is pretty foreign in the first place (why not just include it in the advertised price?).
It's a fixed cost, so they use a fixed fee. If they just added a percentage to the price per night, people who stayed more days would be paying more despite not requiring more cleaning.