I think most knights are about 10% undercosted, with the Castellan, Gallant, and Helverin being 20% undercosted. Stompa shoots similar to a Knight Crusader, but only does about 2/3 as much damage. It hits harder in CC, and can transport. It's got more wounds, but no invul save, so durability is a wash. So I'd like to see it about 100 - 150 points more than a Crusader, and I'd like the Crusader to go up about 75 points. That puts the Stompa roughly at 720, or 200 points less than it is.