or crewed? Makes a huge difference! Bareboats tend to get abused by the less-capable-than-claimed typical charter
"captain", especially a very large (more difficult to handle) cat. A lot depends on how well the boat is maintained by your charter
company and how closely they monitor
the skills or whoever uses it.
We bought a 4 year old Leopard 46
out of the Moorings fleet and I think our price
was around 55-60% of new. Cats are popular so they seem to hold their value a bit better than the monohulls, but maybe that's just in the Moorings fleet where so many of the same monos come onto the market at the same time, causing more price
If you want to be conservative in your financial modeling (you are doing financial modeling, right?), then use 50%, but you should see something better than that after 5 years.