=GOOGLEFINANCE("CURRENCY:BTCUSD") stopped working as of 7/10/2018
You can use a call to the crypto compare api with this command instead:
=SUM(SUBSTITUTE(SUBSTITUTE(IMPORTDATA("https://min-api.cryptocompare.com/data/price?fsym="&"BTC"&"&tsyms=USD"), "{"&CHAR(34)&"USD"&CHAR(34)&":",""),"}","")+0)