na with that amount of cash i wouldn't :s its not really worth the mny if u cant afford it. tho i do suggest using it on abberant specters and dagaanoths.
Also if you ever get Basilisks as a task, I'd advise you to purchase a Dwarf Cannon and go to Pollnivneach Slayer Dungeon to complete it - It requires Smoking Kills to access, but that's easily done It's an insanely fast method, and the charms drop rate is great as well.
<3
Another note:
It's the fastest method possible of killing them - and the drops you collect (as long as you only pick up the more valuable items) can accumulate to be worth more than the cannonballs you use ^_^
I'd recommend buying one if you want to get faster than normal experience on certain tasks. Daganoths and, as Alteranz mentioned, basilisks are excellent for using cannons on.
Not really worth it when you're that poor, but for some task it's real helpful , for example at dagz. But it's v expensive if you only have a 12m bank.
I use cannon but only for certain tasks, i'd say buy one and keep it for tasks like abby spectres and banshees you can cannon them down in the well from smoking kills quest and its pretty profitable on abby spectres because of all the herb/seed drops.
Well, if the 12M your talking about is 'all' you have to spend, a cannon might want to hold off. but as stated above, they are good and will speed things up a lot