(12431/12432) Thiruvanthapuram Central-Hazrat Nizamuddin (Delhi) Rajdhani Express is the costliest Regular Passenger service in Indian Railways.
In Rajdhani Express, 1st AC fare cost is approx Rs 8770 from End to End journey, despite longest Rajdhani service covering the distance 2848 km, it is the fastest and high priority train, takes 41 hours 30 min in non-monsoon time table.
It is operated by Northern Railway Zone in Indian Railways.
This fare is equivalent with Flight Fare From Kerala to Delhi, so one can reach by flight during emergency time, but the scenary, Monsoon journey, food options and getting priority in Indian Railways, no doubt Rajdhani Express offers alternative option.
3rd AC fare might be cheaper option if you feel that 1st AC fare is expensive.
Now coming to the most expensive luxury train in India, then probably Maharaja Express, operated by IRCTC.
Fares range from roughly Rs 5.16 lakh ($4,890) for short 3-night trips up to Rs 26.13 lakh ($26,800) per person for the exclusive Presidential Suite on week-long royal journeys.
The primary route of Maharaja Express is Delhi - Jaipur-Ranthambore and Fatehpur Sikri – Agra- Orchha and Khajuraho-Varanasi-Delhi
Mumbai-Udaipur-Jodhpur- Bikaner-Jaipur- Ranthambore and Fatehpur Sikri- Agra and Delhi