Nice topic! Here's a recommendation for those going to the airshow at Batajnica, Serbia, in September..
Serbia - 20km south of Batajnica is Hotel Vila Apolo in Belgrade.
This hotel is located in a residential area in Belgrade. Very affordable rooms and helpful staff. For only €9 they even come pick you up at the airport, which is a half hour drive back and forth! The rooms are nothing special, but they have everything you need, they are clean and not too small. The only drawback was the breakfast, which was very basic. A supermarket is just around the corner though, and several clubs/restaurants (open 24h/day!) are in walking distance.
If you're interested in visiting Belgrade, it's just a 15 minute (€0.50) busride to the city center - very easy.