I encountered the same problem. What's really odd is that it seemed to occur right after I got my 5th pearl. Up until then the price displayed normally.
Anyone find a fix?
This is a bug that has been reported a few times.
BTW: you need only 5 pearls to buy the cannon, and it is impossible to get more than 14 as long as you haven't bought it.
Please read This page.
Consider trying out the recommendations on the linked page. (uninstall/scandisk/defrag/reinstall and do not patch if you don't need to.)
If that doesn't work, I have provided a savegame just after buying the cannon for both the unpatched and the patched game versions. Make sure you download the correct version.
G-2b, playing the fast game.