As there is absolutely no way to determine average user's cost to own any single component on a boat (every installation is different, all age-related costs cannot be compared, replacement cost varies by size, location, amount of DYI, usage varies by owner, etc.) , I'm tempted to say "who cares-you're trying to pick the fly **** out of the pepper". But, the above-posted numbers are probably as good as the limited sample size implies. And, as some brilliant add guy once stated, compared to the benefits of ownership, "Cost? Priceless!". If you want to practice bleeding before you go boating, dump the generator overboard, and drink warm beer.
If you want to REALLY make your knees buckle, honestly compare the cost of ownership per engine hour of your boat after ALL expenses (purchase, repair, fuel, fixed costs) are subtracted from your proceeds after you finally sell it, divided by engine hours you've accumulated over your ownership. YIKES. But again, "Cost? Priceless".
As with all such "how much does it cost" conversations, YMMV. There's no such thing as "the average boat".
Regards,
Pete