A fast way in diamond is to re-battle trainers
There are two retirees on route 212 outside of the pokemansion on the left side of the left wall. One is a man the other is a woman. Using Vs. Seeker you can battle them. They only have one pokemon each and are relatively easy to beat. You will recieve $8000 each
(if thier pokemon are Lv 40). And if you have a money boosting item such as amulet coin the amount is doubled.
Therefore from 2 short battles you can earn $32000