I've read in a few places that it's possible to take your own motorbike on the ferry to Koh Larn... anyone know what the cost is?

It was a couple of years ago that I did it, but I was charged 100 Baht for the bike to go on the ferry plus the 30 baht for myself. That price is one way. Things may have changed since then.

I think I paid 200 each way last year, its not really worth it especially if the water level is really low. You need about 4 or 5 guys to get it on the ferry, and I am sure any of them don't give a shit if it falls in the water by accident. You can rent a motorbike cheap on the island or get a taxi, the island is really small.