Not wanting to pay an ongoing monthly fee for either monitoring or cell service of alerts really limits the options here.
You can cobble together D-Link WiFi Motion Sensors (currently DCH-S150, about $40/apiece) and control them using IFTTT.com.
iSmartAlarm appears to be a step up, with a wireless hub that integrates a siren and the ability to link and control many devices: extra $30 motion sensors, $25 door/window sensors, cameras, and electrical switches.
Other "self-monitored" systems are built around cameras: E.g., Canary security devices are under $200 and each one includes full HD video as well as motion detection, humidity and temperature measurement, and a siren. Piper is similar, and Novi has a smoke detector built in to each ceiling-mount unit.