No, you cannot afford anything commercial for a coop -- what you want is homebuilt passive solar thingies, such as the 'popcan solar heater' that has been discussed quite a lot on this forum (try 'search') or something of that ilk.
Remember there are two aspects -- one is the capture of solar heat to warm things up when the sun is shining (which is frankly not really when you need heating), the other is havign sufficient thermal mass in the coop (which encompasses a wide variety of design features) to MAINTAIN as much as possible of that daytime solar gain into nighttime, when you *do* want the heat available.
Otherwise all you are doing is amplifying the temp differences between day and night, which is not necessarily good for the chickens.
You don't really need heating, solar or otherwise, in a sensibly-built and well-managed coop around here, though, for most chickens.
Good luck, have fun,
Pat