Having bought my 2002 B36 secondhand, I had the saildrive hull seal changed by the lads in the workshop at Craobh Haven on the west coast of Scotland during the winter from 2010/2011, and I think the share of the cost attributed to changing the seal was in the order of £400. The total bill was significantly more on account of other work also carried out where I had the saildrive shaft bearings and seals changed at the same time due to water contamination of the oil. They did a good job and I've had no problems except for noticing that the alternator connections had been put back incorrectly. That was fixed right away, but it was fortunate that I'd made a careful note before hand of which wires were connected to which terminals, and had checked on completion of the work before running the engine.
The saildrive hull seal is supposed to be changed every seven years, and while I did not see the new seal before it was installed, the old seal which had been fitted from when the boat was new nine years earlier, appeared in very good order. It makes me wonder why Volvo recommend to change it every seven years, but until they suggest it has a longer life then I guess our insurers will take a sour look at any claim if the stated life has been exceeded.