2-3 trillion isk is the current guess.
Current guesses are: ~42,000+ USD, give or take, for the game time alone.
(1 year of play time is about $150 a year, which can be purchased through ISK->Game Time transfers, so assuming all 70 characters had their time payed for by the exploit, thats ($150 * 70) * 4 = ~42,000 USD)
This is not including the extra money (ISK) made through the exploit which was used for other purposes such as high end asset purchases etc.
This comment was edited on Dec 11, 2008, 12:07.
One more thing: As a rough guide, 300 Million ISK (the in game currency) will buy you 30 days of play time. So you could argue that 300 Million ISK is worth about $15 USD in real money.