I would think a DS (dual sport/enduro) in a 400-650cc would be ideal for what you want. Mileage would be extremely good and the bike should be pretty affordable. Insurance could be had on the cheap too!!
Going with something in the 150cc range might be hard to ride (unless you like driving 30 MPH and slower) and the mileage gains may not be enough to justify it when compared to a slightly larger bike.