1. Go to Karamja (it will cost 30gp).
2. Head to the volcano and enter the hole.
3. There will be skeletons and bats there and another hole enter the hole.
4. You will enter a place with high leveled monsters (they won't attackyou.) they range from level 97 to like 131.
5. There you will find 3 mining rocks of gold and silver.
6. Mine a full inventory of gold and store it in the bank (it's in the volcano.) and repeat.

For every 40 gold ore you make 20k and after a while guess what your rich the best place to sell the ore is the Grand Exchange.

Trust me it works!