If your talking about fiber optic broadband then its going to be no more expensive than normal broadband really, BT are just now able to deliver your connection via fiber optic from the exchange, it still gets terminated to a fiber/ethernet convertor, I think they will offer up to 25mb/sec or something at around £40/month, dont quote me on those though!
If your talking about a leased line though thats a completely different story and they can cost you tens of thousands per year.