The worst for maintenance are the Italians. The Desmo valve Ducatis cost well over a grand for a valve adjustment. They have improved with longer intervals, but it still costs when it need be done. Other than the MotoGuzzi, they are a bit more high strung.
Sportbikes are usually more costly due to removal of bodywork and most all are shim-under-bucket that require cam removal to adjust. They go a fair amount of time between, but the first adjustments are the important ones, the valves will actually seat in the head and lose clearance. So buying used one needs to know that the bike had the valves done at least the first two times as required. From there they don't go out of spec quite as fast.
Cruisers and some more basic standards along with about any dual sport single are the easiest to live with. Many cruisers like the Harleys and some Kaw/Honda models (others may be there, but I don't know) have hydraulic valve adjusters and are maintenance free on the most expensive thing that need be done. The ones that do have valve adjustment are usually a bit easier to get to and some use screw type adjusters that are far easier and faster to adjust.
Valves are the expensive part of maintenance. Carbs are the second with balancing when more than one is used, but electronic fuel injection has eliminated that on the bikes having that.
In my opinion, the key thing for maintenance is the valve adjustment (or lack there of) for the bikes you're interested in. Ask about it whenever you look at bikes. Call a dealership and ask about what is needed in a check up and how much it costs. That way you know.
I used to have a Honda CB700SC Nighthawk S, lowest maintenance bike I ever had. Hydraulic valve adjusters, hydraulic clutch, shaft drive - change oil and ride. Find a bike with hydraulic valve adjusters, shaft or belt drive (no/little adjustment and maintenance) and you have it.
KLX650C, Zephyr 550
SR500, Bultaco Sherpa T
Last edited by markk53; 09-15-2013 at 09:12 AM.