What is approximate cost to treat heartworms in an adult dog?

Has anybody ever done this treatment with their dog? I've been told by friends everything from $200 to $1500 for heartworm treatment in a heartworm-positive dog. Does anybody know an "approximate" cost? What is the length of treatment time? I live in Texas.
Update: I'm not talking about preventative heartworm medicine. I'm talking about TREATING IT ONCE A DOG HAS HEARTWORMS.
Update 2: We are considering all factors before we decide to adopt a stray.
Update 3: I'M NOT TASKING THE COST OF HEARTWORM PREVENTATIVE. ~~~PLEASE READ THE QUESTION~~~
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