Just done this mission recently on my new character.
Once you pick up the 'nades drive around block and more will spawn. Rince/repeat till you have full stock = 3k free worth
take the Dominator
use one sticky beetween the two cop cars...Dont detonate
Drive to Vespucci police station and go down the small road from the rear to Riot van. You wont get a warning level. from a safe distance cook/aim a nade and detonate the stickies then throw nade and depart the scene.
head to bikes and cook a nade as you approach, drop as you pass.
Drive to bus and throw grenade at it and head up to hills till you lose wanted level
Head to lester
select freeroam after mission
Go to LSC and put tracker on vehicle
Switch sessions or do a mission /race/whatever
Sell dominator that is now parked in your garage looking all shiny and new for 16k
Pat self on back...You just made 20k for an incredibly easy mission that takes under 5 minutes and all you spent on ammo was 600$ for 1 sticky.