My insurance company, Health Partner HMO, paid 80% of the cost of mine. I was able to choose between Cti2, Townsend Air and Don Joy all about $1100 per side.
I had tweaked my MCL the year before and it was still giving me a little pain but no instability. My other knee I hurt downhill skiing 20 years ago. After seeing the doc and getting MRI's on both knees ($1500 per knee for the MRI's!!!), he told me I had strained MCL's in both knees. He said there's not much evidence to suggest that knee braces do much good as preventatives but he gave me a prescription anyway.
There's a recent discussion on ktmtalk about the benefits of knee braces, or lack thereof. It was started by an MD and, even though it gets a bit heated at times, it's worth a read:
http://www.ktmtalk.com/messageview.cfm?catid=20&threadid=885
I like my braces. I doubt they'd do much to prevent a torsional MCL type loading or help in any kind of high load impact. I do think they help with low speed forces and they for sure help me stand much longer without knee pain. That alone is worth the price to me.