I was looking at this and wondering about "budget" versions.
It has a Price of 75,000, a cost of 37,000 plus 1,000 for the weapon.
It says the thing is a +3 flaming burst ballista, which is a +5 equivalent weapon.
So subtracting the 50,000 cost of a +5 weapon, a "basic" masterwork version would just cost 12,000 plus 1,000 for the weapon, for a final price of 25,000.
Which leaves it as a large animated object with a masterwork bonus to its ranged attack, and 6 HD instead of 4 HD.
So CR 4, between the "normal" CR 3 of an animated object and the "enhanced" CR 5 of a +5 weapon arcane ballista?