DealsDirect are selling the Safety 1st Security Alarm Gate for $49.95, which is hugely reduced from its $139.00 RRP. A quick Google for other prices on it shows that it's about the cheapest you'll find it new. Other places have it available for $128, $129 and $137. Remember to add about $10 for shipping, but it's still way cheaper than even the majority of the eBay ones.
The Safety 1st Security Alarm Gate gives peace of mind that their children are safe from falling down stairs or going into rooms you don't want them to. It has a handy alarm feature that sounds in crescendo when the gate is left open.
It also has a SecureTech indicator to let you know that the gate is securely locked. It's a metal construction gate and is hardware mounted. For ease of use, it has a quick lock and release handle and is 74 - 107 cm wide and 71cm high.
The alarm requires 4 x D Batteries.
I think for my first child, I would have thought this just a paranoid bit of baby paraphernalia, but now with my second, I find my oldest is old enough to open gates and doors on her own, and the baby often toddles off after her. I can definitely see the use of a gate like this, both for reminding her that her sister is too small to know what's dangerous and she needs to remember to close the gate, and to let me know when she's simply forgotten!