When did you last have a radiator or water pump fail? You are meant to change coolant every 2 years, and most bikes it's a ten minute job costing half of a £20 bottle of 50/50 premix.
Air/oil cooled bikes typically go through the oil quicker, and that's a bigger, more regular cost. Depends if this is actually factored into the service interval though.
Air/oil cooled are also harder to keep clean with all the fins, and can therefore tend to corrode earlier.
Typically though I will grant air/oil cooled bikes use cheaper plugs, but I only have experience of XJ600 vs CBR900, and the price difference isn't that big.
Ducati engine families
Air-cooled = two valve, 8 shims, single OHC
Water-cooled = four valve, 16 shims, DOHC
Both packaged in a similar overall sized head, ones got water cooling jacket, pipework and radiator, other hasnt.
The extra gubbins make it all very cramped, belt change on a SOHC is easy, on DOHC its a pain.
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