I can tell you one reason why codestones were going up and up and then suddently fell: The
NCW 1-player war. A bunch of BD guilds are participating in this war (all the biggies -- Battlers Pride, Celestial Dawn, Phoenix Warlords, Cursed Exiles, etc.), and there are 750+ battlers signed up. Everybody's been training like mad to be ready for it, and now that it's actually started (just over a week ago), they've stopped training because battlers are divided into 24 different leagues by stats and people don't want to move up into a new league, especially if they're at the top, score-wise, of their current one.
Of course, the Altador Cup also plays a part -- a lot of people misunderstood the editorial a few weeks back and thought we were getting a June war. When they saw we really aren't, they stopped hoarding the stones. Prices are now pretty steady around 3900-4000 for everything except Bri (3200-3300) and Main/Eo (6500 or so, sometimes less).
Also, Lutari Island supposedly has its own training school, which will use a different currency. Look for codestones to perhaps drop in price even more once that's released and people start training there.