Dear Costas,
I don't really know the prices.
You should check the online system we propose on
http://www.greeka.com/greece/greece-ferries.htm
You will find info about prices and schedules.
Have a great trip to Paros. And Ios will be a great experience for you. It was for me many years ago.
Santorini is around 4-5 hours away from Paros with the normal ferry. There are some high speed ones that will take just a couple of hours. Santorini is THE most beautiful island of all. But I would recommand it for couples more than for friends interested in nightlife.