hmmm let's see...I'm afraid I can't help you much with these two questions...
As far as camping goes, well, there are campsites pretty much everywhere on the route i suggest, but a source of information that groups them all? sorry, no idea. As far as camping prices go, well again I can't help you as I wasn't carrying a tent when i went so I didn't consider them, but take the usual rule; Chile is more expensive than Argentina, and the farther south in Argentina you go the more expensive stuff becomes. Camping inside the park next to El Chalten is free, and in Torres del Paine, some places are free to camp and some are not. Those that are free are further away and have no facilities and in summer they get rats, so don't leave any food where they might smell it, and that includes the tent, as they have been known to chew their way inside to get at food they smelled.
As for Coyahique, if you go down that way, you'll have to cross into argentina to get down to torres by land, or alternatively I think there's a ferry from round there too, but I have the feeling that Coyahique is quite isolated and that public transport out of there except by air is fairly scarce. But then again I've never been so I don't really know. I have been told that the Area is very beautiful though.
Don't know what else to say so...
Good luck!
John