it gives a gud 13-14 km/L in City, CNG goes upto 130kms a full 55L cylinder.
Maintainence, if ure driving on petrol theres no maintainence besides oil and filters being replaced timely,
On Cng you could face misfiring, and weak ignition coils.
If u drive on rough roads, Baleno has a lower arms (keinchiyan) issue they need to be replaced, Bushes cannot be just replaced. (cost too much i guess 18k for Genuine ones, and about 5k for Pakistani/Taiwan ones)
Spares are not costly, But not as cheap as a mehran/alto/coure/cultus either. They fall in the range of Civic/Corolla/City spare parts prices.
Other then that its perfectly fine car. and the best one in its price, you dont get a 2005 diggi walli garri in that kinda money these days, Plus CNG is a bonus too