The Rebel 300 mentioned gets 71 miles to the gallon. The best I've found in that class is the Yamaha XV250 which gets 78 mpg, more (up to the 90's) if you tweak the gear ratios. Liability insurance is negligible.
There are plenty of motorcycles that do much worse, especially once you get larger -- but this seems to be improving significantly.
Agreed that scooters are typically more complete in their functionality. Some things that I took for granted aren't ubiquitous such as fuel gauges, oil pressure sensors, and center stands. Windshields and luggage racks aren't standard. On the XV250 in particular, it takes some resourcefulness to get saddlebags properly supported.