Given that Wal-Mart has a sizable presence in other countries, they would have to increase the wages for far less than two million workers. Hell, they have locations in countries with higher minimum wages, and yet they still find a way to make money.
Yeah, if we assume US-only (fuck you, ASDA workers), then the costs come down to $12.6bn, from the $42.0bn claimed (minuses: 30hr work week, 700,000 non-US workers, added dollar from shite maths in the original).
So guess what? I saw an updated version where they subtracted that extra dollar per hour, and they STILL fucked up the math. Seriously, even with the correction, they're still assuming that all 2 million employees are Americans working 8 hours days, 365 days a year, and earning hourly wages (I do believe some employees are salaried). This isn't even taking into account the extra income Wal-Mart would get from their employees having more money to spend - as Magus did point out.
To be fair, the figures would be higher than the 30 hour workweek calculations, but that's only because Wal-Mart gives holiday pay that's equal to the person's current wage multiplied by the number of holiday hours they work. Even so, that's a fraction of the year and a fraction of the workforce. It also gives said employees more disposable income of which some will be spent at Wal-Mart.