How much fuel does it take to start/land an aircraft, compared to normal flight?
The fuel consumption or fuel burn rate is dependent on the aircraft type, size, engine configuration, cruise altitude, temperature, thrust setting and other factors.

In general: The larger the aircraft, the less economic do short flights become due to weight/size and fuel burn on takeoff/climb.

Here are some values I found for the CRJ2, the source may not be entire citable:


  Approximate total burn pounds / hour
  
  Taxi: 700 (Both engines and APU)
  Takeoff: 6000-7000 (depending on temperature, elevation, reduced/full thrust, etc)
  Climb: 6000-3500 (reducing as as altitude increases)
  Cruise: 2500-3000 (depending on altitude/weight/etc... subtract 500 for long range, add 500 for high speed)
  Holding: 1800-2400 (depending on altitude/speed)
  Normal descent: 1500
  Idle descent: 600
  Approach: 2000 (fully configured)  


(Source: www.airliners.net - Author: Flyf15)

You can also read the following related questions to understand how altitude affects fuel efficiency:
Why do jet engines get better fuel efficiency at high altitudes?
How does flight duration affect the hourly fuel consumption in heavy aircraft?