Technically, cutting it down is considered breaking it, thus preventing the magic from working... but rule zero can easily take care of that. Also, I'm trying not to laugh at the mental image of a pea sized bead shooting out of the tip of a colossal sized weapon.
OT: Could you clarify "d? number of rounds". Without and exact indication on how often it can be used, pricing will be difficult. With that out of the way, lets take a look at a few items that can cast fireball.
(1) Necklace of Fireball - In the text, it states you can purchase individual, single use spheres for 150 per 1d6, putting your 8d6 at 1,200gp per casting.
(2) Staff of Fire - While its highly unlikely that an 8th level player would get their hands on one, lets alone only use it to cast fireball, if they did use it just like a wand, they'd be spending 355g per casting.
(3) Wand of Fireball - Keeping with the 8d6, we'll make set it to 8th level caster, so we have 375*3*8 = 9,000gp. If you look at the price per charge, you're paying 180g per casting.
(4) Scroll of Fireball - Using an 8th caster level (12.5*3*8) we get a 300gp price per casting.
(5) Blast Globes from the MIC provide a 400ft range 10d6 fireball once per day for 8,000gp. Nowhere near exact, but if lets fudge some number. Dividing by the caster level 10 and then multiplying by caster level 8 we get (8000/10)*8 = 6,4000gp. Honestly that feel rather cheep... especially compared to the custom magic item rules (3*8*1800 = 43,200gp).
Taking all of these examples into consideration:
[A] If the "siege" weapon has a limited number of casting and is non-rechargeable, I'd set the value at 2000gp + (300gp per charges remaining). This version should have at most 50 charges.
If it has a limited number of casting and is rechargeable, I'd set the value at 2000pg + (1,200gp per charges remaining). This version should have at most 10 charges.
[C] If it has a once per day use, I'd charge 8,400gp.
[D] If it has between 2 and 5 uses per day, I'd charge 2000gp + (3,20gp per use).
[E] If it has unlimited uses per day (even with a cool down in the hours range), the PCs should not sell it. It would be in their best interest to invest in Knowledge engineering (their own or paying for a NPC's services) to construct a siege platform to house and utilize this weapon. Of course this would completely change the nature of the game you guys are playing. If I had to put a price on this, I wouldn't go any less than 43,200g
If you're wondering where the extra 2,000gp is coming from, I figured that was a decent price for the PCs to tack on for safely cutting the weapon down and hauling it to whomever ends up buying this monstrosity.