Honestly, if you just need a simple, non-invasive entry-alert system for your apartment, you can grab it at radio shack for cheap.
All you need is a small, yet loud, electronic bell or buzzer , preferrably one used in alarms. Then you pick up a simple magnetic switch.
The magnetic switch remains ‘off’ while the two magnets are brought together. So you’d put one at the top of your door, and one on the door frame just above it, and connect it to your buzzer.
When the alarm is engaged and the door opens, the magnets separate and the alarm goes off.
Simple, cheap, effective and very little effort and installation. That’s the low-budget route. And since it operates on a AA battery, it’s ‘wireless’...technically.