Wouldn't that depend on how much you can find the games for? Then multiply by however many toons you want to run?

Cheapest was probably during the christmas sale, when they were selling vanilla + tbc + wotlk for a total of $20 then plus cata for $40 = $60 to fully upgrade one account. So $60 - $240 in increments of $60 depending on how many accounts you want to run.