After the mission 'Truck Hustle' you will unlock the assassination missions and these are pretty simple . there are 9 and you usually get paid between 5 - 10 k a mission these are pretty easy cause most you should be able to complete with a sniper rifle and some are chase orientated but pretty easy.
List of assassination mission:
1. Bailing Out for Good.
2. Dead End.
3. Derelict Target.
4. Hook, Line and Sinker.
5. Industrial Action.
6. Migration Control.
7. R.U.B. Down.
8. Taken Out.
9. Water Hazard.
Short guide for assassination mission:
http://www.gta4.net/high-end-assassinations/