You need to have a from and to currency like this:
=GoogleFinance("CURRENCY:USDBTC")
For historic close price use:
=GoogleFinance("CURRENCY:USDBTC","close","07/07/2017")
If you want only the price returned use:
=iferror(index(GoogleFinance("CURRENCY:USDBTC","close","07/07/2017"),2,2))