I am actually in the middle of doing this for my other forum. We are letting anyone that wants to be in the calendar send me pics, I have them posted in a Gallery. We ( members) will be voting on the cars that go into the calendar. That way anyone is fair game, well except for me and the other 3 mods, lol, I have decided to let us 4 have a month, since we take care of it.
Also I may end up making it a 14 month calendar depending on how many more people are interested., So I will be able to have 15 cars total, Front cover then 14 months. So far I have a quote locally for a 12 month calendar and it would cost about 17$ US. A member of my forum did find me a link online that would cost 12$ US per calendar. The quotes I got were on a QTY of 20 ( that is how many people have shown interest in it so far. The next price break for me would have been at 50.
So you are very likely to be able to find a place to do them for at least that cheap.
Here is the layout that we are probably going to be using ( pretty close anyhow).