DFDS ferries (to Norway / Amsterdam / Denmark) - £5 each way
Northlink ferries (to Orkney and Shetland) - free
CalMac ferries - usually between £1 to £3 each way, depends on the route.
Have Clamac changed their policy? When we went to the Outer Hebs Windyland in 2005 the bikes went free. We got a five ferry ticket for something ridiculous like £36 and went out via Oban and returned to mainland via Skye to Mallaig.
When I used CalMac (2004) there was a charge of £1 to £3 for bicycles on a single or round trip ticket. If you bought an "Island Hopscotch" ticket, which was a series of crossings on one ticket, bikes went free which made the Island Hopscotch ticket a really good value instead of just a good value. There are a wide variety of Island Hopscotch itineraries available, with no time constraints.
BA: free of charge, and they've taken good care of my bike in the past. They now require a bag according to their website.
Lufthansa: excess baggage charges apply, but they're pretty vague about how much. When I flew with them (2006) they made me deflate my tires. When I complained by e-mail they dropped that requirement, they have since reinstated that requirement.
Aer Lingus: €35, €30 if you pay online in advance.
Any US airline: fairly massive charges to bring a bicycle.