If you're going cheap, buy a Cadre Fireblade HQ and a small squad of firewarriors (less than 150 points total) and then a riptide. The whole package should come in well under 400 points total.
Arguably, you could go Ethereal + Kroot and a Riptide and sit at about 300 points, but I don't recommend that as the Ethereal dying is bad for you and, when stuck with a bunch of Kroot, he is almost certainly going to die.