155 again seems a little low. Open topped, I presume your going to want to assault the turn it lands?
So what we now have is something that could be 7 inches wides, that may scatter a bit, that can cause strength 8 ap 3 wounds AND then dump out 30 orks or more than likely 3 killer cans. Take that to normal extremes and Turn two sees a mess right in the middle of the oponents deployment zones at least potentially.
so 155 Sounds like a steal. I'd like to see it play tested.... I think bare minimum, your looking at 250pts.
or to make it a bit more Orky we could try this, everything under the template suffers a strength 8 ap 3 hit and the rock suffers a strength 8 ap3 hit for every wound caused by it landing. Passengers suffer 3d6 str 4 ap - hits.
Cheers,
Alan