I've traveled this route several times and the best advice is to avoid booking on the spot at the airport or station you'll likely overpay. For a reliable Taxi Service in Delhi for Outstation (https://indiatourtaxi.com/), you should look for companies that offer fixed 'all-inclusive' rates (including state taxes and tolls) so there are no surprises mid-journey.
A few tips for outstation cabs from Delhi:
-> Check the Permit: Ensure the vehicle has a 'Commercial T-Permit' for interstate travel.
-> Innova vs. Sedan: If you're heading to the hills (like Shimla or Manali), an Innova is worth the extra cost for comfort.
-> Driver Verification: Always ask for the driver's details at least 2 hours before the trip.
I personally found India Tor Taxi to be quite transparent with their pricing for outstation trips. They specialize in long-distance routes and their drivers are familiar with highway safety protocols.