The Marans you have been seeing that look similar to doms and rocks are the cuckoo Marans. There are several different color varieties of Marans, however.
Price depends on a lot of factors -- such as quality, age, location, the individual seller, and so on. You can even buy cuckoo Marans at some of the big commercial hatcheries these days. I wouldn't RECOMMEND doing that, but it can be done.
If you are interested in Marans, the most important thing I can tell you is to DO YOUR RESEARCH BEFORE YOU BUY. It is very very easy to get bad quality birds right now. In order to get good birds and a good deal, you need to do the homework first!
If you'd like to learn more, ask questions here on BYC -- or join one of the several yahoogroups that are dedicated to Marans. I hope to see you around!