It can also depend on local costs which can vary by area. Here in southern BC, Canada on the west coast, I spent around $10,000 per year without leaving the dock (moorage, insurance, etc.). On top of that, doing almost all of my own work, I spent another say $1000 just for supplies like oil, filters, impellers, etc. just for routine maintenance like oil and filter changes, fuel filters, impeller change, etc. every year. Then, there would be larger expenses every few years like dripless shaft seal maintenance, cutlass bearing, prop repair, replacement anchor chain, haulout, bottom paint, bow thruster maintenance, etc., etc., etc. Labour costs when needing professional help is well over $100 per hour, often starting from the shop and not stopping until they arrive back there. Also, on an older boat there will be things that just plain need replacing due to age, and some items will fail requiring repair or replacement.
I don't agree with the 10% of purchase price estimate due to the fact that an older less expensive boat will need more put into it than a newer more expensive boat, and the 10% idea indicates the reverse.
To keep an older boat in good condition is not inexpensive, and could easily run over $20,000 per year on average (including moorage, insurance, etc.) and in my experience in some years even more.
