Do you mean setting bio gas from cow dung and use it direct commercial purpose. In my view direct commercial purpose is not viable.
You may use the bio gas as energy source for your allied activities like
1. To replace LPG gas for cooking
2. To operated engine to irrigate your field
3. To use the biogas to pasturize your milk and make value added products like ice cream, milk cakes etc
4. May convert biogas to electricity, but its economic profitability is questionable
4. Also use the bio gas plant byproduct, the slurry to produce vermi compost.