Who would you even SEND a billion dollars to if you wanted to buy that much bitcoin? Gox? You'd log into your gox account and it would say 1,000,000,000? And you would click "Market order"? Not happening. Not now, probably never on gox.
Whale MO:
Market can't take that and the risk is too high. You'd start by cracking through a few ask walls (because you know your demand is going to push up the price so you might as well be the one that benefits) and then proceed by transferring a few million USD worth a week (descending as the market thins out) and buying on multiple exchanges simultaneously whenever reasonable ask walls start to build up. You'd be willing to buy on exchanges that are 10-20% higher than others because you know your purchases alone will drive supply/demand curve enough so a 10% premium will be over taken in a week or 2, if that is where the liquidity is.
By transferring weekly, you never have too much value on the exchange and you eat the vertical portion of the ask curve, which gets you the most coins without driving the price much (you want to keep the price low for next week). You'd buy BTC right away to minimize the time $ is on the exchange... so early in the week is likely.
If you have left over $ for the week -- that is, there weren't enough asks, you put up a wall maybe halfway back to entice big sellers to move BTC onto the exchange (its fine if they don't sell into your wall. if its put up as an ask, you'll get it next week). If you spent your weekly transfer, then no walls and no activity for a week -- to let the price drop down as far as it will go. This is a better choice for a "whale" than putting up an ask wall b/c bitcoin is now officially $, the exchange knows who you are, and you have no interest in being prosecuted for market manipulation over maybe 5%. And especially when it puts your hard-won BTC at risk of purchase by a surfacing whale.
Pump MO:
If you were "pumping", you are doing it differently; you want to maximize slippage. So you'd buy in much bigger gulps -- the further you push along the curve the further your dollar pushes up the BTC price. Possibly even right after someone else's big buy (remember a bit of a loss in buys don't matter much -- if you are pumping you have a much bigger stash you are prepping to sell) And you'd probably do it on one exchange only, relying on arb and trending bots to buy up the others. Pumping like this would require a protective bid wall near your final price to psychologically cement in the new price. You would never leave an ask wall half bought, there's no pump for your buck. You'd put a bid wall up safely away from it and hope other eat into it while you transfer more $. And when you did eat it, it would be the wall + the flat behind it...
hmm... which do you see last few weeks?