theres a mission with 1000 where you can chose to keep it or give it up. also like stated hunt and capture weeds and shit. certain spots pay more for "exotic" goods. i think stuff in mexico go for better price in tall trees and such.
MTT, i dont think it counts as cheating but buy a survivalist map. it shows every plant on the radar for like 20 min. after you level high enough i think it doubles in time. just find what spot the plant grows around, go there and turn that sucker on