The ETH pre-launch allowed people to exchange Bitcoins for ETH at a determined value.
What was this exchange rate in USD?
E.g. 1 Bitcoin got you X Eth, which was worth about $Y.
The Ethereum presale took place from Tuesday, 22 July 2014 to Tuesday, 2 September 2014 (42 days).
22 July to 5 August 2014: 2000 ETH = 1 BTC = 572-632 USD 6 August to 2 September 2014: 1999-1337 ETH = 1 BTC = 592-471 USD
USD prices are the range in the given period of 24 averages on multiple exchanges, Source: Quandl
For the linear decrease of ETH in the second period, refer to this source:
The price of ether is initially set to a discounted price of 2000 ETH per BTC, and will stay this way for 14 days before linearly declining to a final rate of 1337 ETH per BTC. The sale will last 42 days, concluding at 23:59 Zug time September 2.
More information about how it turned out: https://blog.ethereum.org/2014/08/08/ether-sale-a-statistical-overview/