Yooo man!
The txt file will work for sure.
But it's completely possible to do the database thing:
SQL has the "-" operator, so you could do something like "UPDATE * where `tier` = $tier VALUES `chances` = `chances`-1". you just need to call that by PHP or something. That's the ideal if your game will be played in multiplayer with some people at the same time, so it doen't conflict any information!
Hope i helped you someway, if not ask again 'cus maybe i didn't understood your point or if you need more information about my method. But please don't chance post for that, i still wanna see what is your solution at the end.
-- Yuri Seiya from Utopic Blades Team --