It all depends, like most have asked, we need more details.

A 5.5ft normal boa may advertise anywhere from free to good home to $100 to $200 to $300 depending on the quality, health, sex, weight, if he/she is a proven breeder, etc, etc, etc.

More than likely these classifieds with tank and all are probably a family pet that got too big and they want to get rid of it. I would not take the fish tank or all the accessories depending on what they are. In a case like this you could probably offer them $100-$150 for the snake and lose the extras.

Now whether he/she is worth $150 or $300 depends on a few things and how much it's worth to you.