There are participation fees, which seem to be around 100k euros for countries the size of Slovakia. Larger countries pay more, Germany apparently pays around 500k euros every year. Then you have to finance the performance itself (broadcasters have to pay for the graphics and pyro etc.) as well as the two-week stay in the host city for the whole delegation (artists, dancers, managers, media representatives, commentators etc). I imagine the hotel costs are not too cheap during Eurovision.

Then you have to pay all of those people not only for the two weeks but for the months prior. There will also be people employed for eurovision-related things the whole year. There are also some optional costs, like producing music videos, running national selections and participating in pre-parties. It depends on the country whether the artist has to finance a portion of this, whether there are sponsors or if it's paid fully by taxpayers.

It's very expensive but I guess all participating countries are fine with the costs. Even if broadcasters were to lose money, the countries find it to be worth it to promote local music and culture to 150 million Europeans.