About twice a year I see a monitor for sale in "Latitude 38" here in San Francisco. Twice I called as soon as I saw the issue and the monitor had already been sold. One person I know who bought a used monitor ended up within $500 of the price of a new one by the time he had the mounting tubes for his boat fabricated.
I had a monitor on my NorSea 27 and really liked it. It steered the boat all the way to Hawaii with the exception of a brief 20 minute experiment with the autopilot.
The only negatives I noted were that the monitor on the stern made stern anchoring very difficult and my plans to use a small autopilot to operate the vane while motoring were nixed by the monitor folks who said that the prop wash put a lot of stress on the oar.
Im having a new boat built and plan to put the capehorn vane on it. This elegant device only extends 4" beyond the back of the boat and connects directly into the boats steering system. The oar that goes in the water is custom made for the size and steering characteristics of your boat and is only about $180 to replace. The oar on a monitor is $1000. The capehorn folks endorse the idea of using a small tiller pilot to operate the vane and even provide the below deck attachment point. Their web site is www.capehorn.com. They have several models for different sized boats and they are cheaper than monitor, especially if you consider that monitor sells you a $200 bag of spare parts and capehorn does not need any.
The capehorn vanes can be highly customized for the boat (taller vane, differen oar, different mechanics) so buying a used one might be more complex than buying a used monitor. But, the parts are cheaper so making a used vane fit a new boat should cost less.
The monitor worked fine and, like I say, I was happy with it. But, the capehorn has a good reputation and overcomes most of the problems with the monitor and is cosmeticaly superior.
Also consider the cost of berthing the two and 1/2 feet of a monitor vane versus the 4" of the cape horn vane. My slip rental went up about $15 per month when I added the monitor. slip rates were about 4.50 a foot at that time...now at 7.50 a foot the monitor would cost over $22 a month. A lot of people remove the monitor when they are not going cruising.
The monitor folks asked me how I would feel about trying to fix problems with the steering attachment on the cape horn when the weather was bad. I think I would rather be upside down in a lazerette than upside down over the stern of my boat. In my experience the vanes are so reliable that the probability of a failure is small. Also the capehorn control lines are out of the sun and should last much longer that those on the monitor which are always exposed to the sun and weather.
A windvane is a wonderful addition to a boat. Its silent and leaves all your spare amphours for cooling the beer. It does not get tired or complain and the harder the wind blows the better it works. Getting it to work under all conditions also teaches you a lot about sailing your boat in a balanced state.