This is really a very simple system... I played with it for about a year to get it to work like I want it to...
Starts with a very affordable motion sensor I buy off ebay... about $1.00 each
http://www.ebay.com/itm/170896482447?_trksid=p2060778.m1438.l2649&ssPageName=STRK:MEBIDX:IT
The siren if very loud... just a fancy low voltage siren I get off ebay, about $2.50 each or so
http://www.ebay.com/itm/KDQ14-Mini-...534?pt=LH_DefaultDomain_0&hash=item43cd307bce
The key feature is this little transceiver, radio RFM12B... this thing allows you to send a signal to a monitor inside the house or hotel. Only has a range of 100 yards or so... low power module. You can pick these up closer to $5.00 on occasion. Again, I like this as you can carry a monitor with you when out of town. Anybody messes with the trailer in the hotel parking lot, you'll know about it.
https://www.sparkfun.com/products/12031
You need an arduino chip ($5.00) and a 28 pin dip socket and a handfull of resistors and capcitors, LED's (maybe another $5.00) and the circuit boards I have put together. I order the boards from this place in China (of course), where I can order small quantities... they can manufacture these and ship them to me inside of a couple of weeks... pretty crazy. The boards are probably $6.00 or so in small batches... this stuff and a little time to solder it up (some electronic solding experience required)...
So that adds up to around $25.00 per alarm... the receiver/monitor is a good bit less... less parts.... and, the radio and monitor is optional. You can just put the alarm together without the radio and monitoring feature...
I just started working on the cabinets in my new trailer, so, it will be a little while before I can post any installed pictures.... couple of weeks.