Some people get a 25% price reduction on house/renters insurance for having an active security system. Its worth asking. Also, a 95-cent Sliding Window Lock with Thumbscrews (2-Pack)-U 9802 - from The Home Depot is a cheap security measure!
Prime-Line Sliding Window Lock with Double Thumbscrews
The Defender Security Sliding Window Lock With Double Thumbscrews securely locks windows in a closed or ventilating position. The lock features 2 thumbscrews for additional gripping power....Learn More
Wedgit White Maxi Twist Tight Adjustable Sliding Window & Door Lock extends from 25" to 42" - http://www.thepuppy.org/wedgit-white-maxi-twist-tight-adjustable-sliding-window-door-lock-extends-from-25-to-42/
Economy Aluminum Sliding Window Lock with Thumbscrew