It's legal where I live, it might be different depending on your location.
But we have one at the pet shop I work at and it's an alright pet. If you are really into them, I suggest getting it young. Since they may be hard to find, they can be found anywhere between $89-250.
As far as a cage goes, you would want a 30 gallon high at least. maybe move it to something bigger after that, but you would also need a heat lamp.
They can eat crickets, meal worms, and pinky mice.
Ask around to find when the next reptile show is. At reptile shows, you can get supplys and animals MUCH cheaper than at any pet store.
Frilled lizards only extend their frills when they get upset. (I have seen it move when they eat and do other activities though)
They are relatively easy to take care of. If you can't find one of those, I suggest getting a Bearded Dragon.