If low running costs are all that matter, and you can live with the official 143 mile range (likely lower in the real world), then it's worth considering the all-electric B-class, tagged B 250e. Saying that, at a minimum price of £27,275 (including the government's £5000 plug-in car grant), it's not cheap to buy.
The diesel engines are probably the best option for most. The B 180d and B 200d return combined economy figures of 68.9- and 65.7mpg respectively, though when fitted with an auto they return an identical - and rather impressive - 70.6mpg. The B 220d (auto only) isn't far behind at 67.3mpg, though that drops to 56.5mpg in the 4Matic four-wheel drive car.
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The petrol versions shouldn't be immediately discounted, however, as they're cheaper to buy in the first place and will suit low-mileage drivers. Both B 180 and B 200 return 50.4- and 52.3mpg when fitted with a manual and automatic gearbox respectively.
Emissions ratings vary from as 104g/km in certain versions of the B 180d and B 200d to 132g/km in a few of the petrol cars, which means annual road tax of as low as £20, or as high as £130. Insurance ratings are 15 to 24 and Mercedes-Benz allows buyers to spread the cost of fixed price servicing with its Service Care offering.