The Alto. Capable of 15-17 km/l in the city, and 30 on the highway.
Oh, you want something
nice?
The Celerio is probably the best at the moment, It can nearly match the Alto in the city, and is better on the highway thanks to the aerodynamics. The i10 is great in MT form, but just okay in AT. I am generally satisfied with the AT economy, but some city users complain.
There's a new Picanto coming, and it'll have a small 3 cylinder like the Celerio, which means it should be almost or just as good as the Celerio, but it'll probably cost more.
With your budget, you cat get a secondhand 1.3 MT Honda City that will be as good as or better than many of see cars in terms of fuel efficiency. For 500k, you can probably get one of the last-run Second generation cars with some warranty left. And it will have much more room to boot.