Right... well Duke Freedom counted about 500b in 2005....
http://www.tip.it/runescape/index.php?times=6
Rares are arguably the biggest indicator of the inflation rate - and rares have increased in value by about 2000% since then. Adding the effects of the increase in RS population (x5 or so), I'd say the figure lies somewhere around 10000%... Giving my estimate of there being roughly 50 trillion gold pieces currently circulating within the economy.
The article quoted in an earlier post is somewhat deceptive. Many of the player-player trades will be items-items, and most will be cash-items - I'd say the largest amount of actual 'gp' you could draw from that figure would be about 25%. The Wilderness/Staking figure will again be mostly items.. about 10% of the given value, and the Grand Exchange figure would be exactly half. Also remember that a lot of that will be repeat trades, and the repeat flow of money, (i.e. if someone trades another person 1 billion, and they then trade it back, that would count as '2 billion' traded).
So based on my estimates there's about 5000B floating around in the RS economy in actual 'gp'. Obviously there is several times more than this in items, but in reality the items are worth only their alch price in cash, as cash in never created when trading an item.
I'm tempted to do what Duke Freedom did all of those years ago, and put together a thread with a more rigorous estimate on the cash total in the game.