I have a PYI PSS Dripless Adapter on my C310. Its 4 years old and has seen over 1200 motoring hours and 10K nautical miles without issue. I am hauling out this year and I will replace the bellows. I am doing this as preventative maintenance and the part was $80 and we only haul out every 3-5 years in the Caribbean.
As far as cost, the first time I did it they were high. But I changed the shaft, the transmission cuppler and the cutlass bearing. I did all the labor myself and it probably ran around $900. For this maintenance, all I am doing is the $80 maintenance kit from PYI and new hose clamps.
Make sure you read this post from MaineSail if you are going to do this yourself. Read the article even if you higher it out so you can get an idea if they did it right.