Finney is a denomination of Ethereum (What is Ethereum?), 1000 FIN (Finney) = 1 ETH (Ether)
Next to the tip button on a performer's page, click "What is ?" and it will show you the exchange rate.
The price of Ethereum is determined by the market, and can fluctuate from day-to-day, and even hour-to-hour.
It is strongly recommended that you have an exchange account so as to have an outlet to exchange your ETH for USD when need be. Some would tell you to "hodl that shit", which is fine, but there's no shame in paying the bills! :)