[Demo]

A good method to make money is to kill the cockcroach soldiers in the stronghold of player safety. Per trip you could be looking at about 500k (depending on how long you're there for).

Note that trip length (and therefore money made) depends on your combat level, constitution level, your armour and what food you bring. Some items they drop (Addy ore, mithril ore, and a variety of runes) stack so inventory space isn't really an issue.

Here is a link to the cockroach workers stats, what they drop and so on.
http://runescape.wikia.com/wiki/Cockroach_soldier

Hope it helps!
-Demo (Runescape: Demo2gbraah)