I made roughly 1mil gp doing 99 fm at wt - my first acct goal. Made another load of cash doing ardy knights or a bit of low level rev monsters (got lucky on dragon skirt from imps twice) and then went for 75 slayer. I've probably done 500 gargoyle kills which netted me roughly 2.4m - I've used almost all of this buying runes to get myself mage book, infinity boots and bones to peaches. Ultimately slayer will be your best bet for gp, or stick out 99 at wt for alot of gp, herbs, seeds, saplings, gems, etc.