Most service items happen on a 5,000km (3,100 mile) interval, such as oil & filter changes and valve adjustments. The cost will vary greatly with whether or not you can do the maintenance stuff yourself. The valve adjustment is fiddly to do, but at least they are screw-type adjusters instead of shim under bucket as on many Japanese bikes. The oil change procedure is also fiddly... you have to replace TWO filters (one on the frame and one inside a cover in the engine) and five sealing washers with each oil/filter change, but it only takes 2 litres of oil. The OEM KTM oil filters are pretty expensive, but there are K&N high quality substitutes for the OEM filters at about 1/2 the cost.
As far as reliability goes, they probably aren't as care-free reliable as your average Japanese "appliance" where you can ignore maintenance items, but they are far more reliable than some Italian marques I can think of. KTM has quite a winning record in motard, mx, and enduro racing, and reliability sure comes into play there. The KTM LC4 motor has been around a long time and has been incrementally improved over the years to the point where by and large they are very solid and reliable.