Travel policy challenges
Travel policy is one of the major headaches for many companies. Most people are generally honest, but there are always a few bad apples who will find ways to exploit the system.
Per diem allowance system
Many companies have found that having a set per diem allowance for employees to cover accommodation, meals, and incidentals such as dry cleaning and taxis is effective. Usually, the company will organize and pay for the travel, e.g., flights, trains, etc. With this system, the employee has to book their own hotel/accommodation, etc. They are responsible for making their allowance stretch to cover the costs.
As various cities and towns have differing accommodation costs, the allowance has to take account of this. For example, Delhi hotels will be more expensive than those in a small town. So, you might have a higher per diem for employees who need to go to Delhi, Mumbai, etc. The same applies to meals, as food may cost more in bigger cities.
Potential flaws in the system
Now you can probably see the flaw in this system. Give an employee x rupees for their trip. If they are smart enough, they will find a hotel for y rupees, eat street food, and generally find cheap ways to get around. In the end, they may be able to save some of the money they were given. I know this because I have done it.
However, some employers find this is easier than trying to get employees to bring back receipts and account for their expenses, particularly those who overspend without regard for the company's money.
Importance of a comprehensive policy
As Dinesh has said, whatever you do, YOU MUST cover all the bases and make the policy as comprehensive as possible. Once you formulate the policy, you then run a training session to make sure everyone who needs to travel understands the policy to the letter. They also need to know that transgressions will be dealt with severely. You must have penalties in place and strictly enforce them to warn others.