A bit off topic, but when you buy that many shares of a public company can you negotiate a discount? Is the trade done directly with Apple, or does Berkshire Hathaway just buy it through Nasdaq ?
After that happens market participants can sell those shares to each other based on supply and demand. As demand for a companies shares rise, prices rise and vice versa. The secondary market is what you and I as individuals think of when we talk about buying and selling stocks. There is no real discount that Apple could offer Berkshire because they aren't selling the shares, market participants are (individual investors, brokers etc). The trade would take place as a complex series of transactions on a variety of exchanges. Sometimes Nasdaq, but other times on exchanges like ArcaEdge, Bats, Direct Edge, NYSE and others.
The complexity is that if you dump 1 Billion dollars into the market to buy x number of shares of AAPL you will drastically increase demand and move the price considerably, so the trade must be executed as a series of transactions over a given time period.
Now if one public company buys another public company in a merger there is a set share price that is negotiated but that's outside of the normal process of buying and selling stocks.
It all comes down to whether both Apple and Berkshire can agree on a price and cash flow that is better than buying/selling on the open market. This would probably not be a great discount for the buyer. Other companies would probably still ask Goldman to broker the sale (at a lower fee) and take care of regulatory compliance, but both AAPL and BRK like to maintain in-depth knowledge of the intricacies of the stock market in-house, so they may just do everything themselves.