Okay i've got like 20mil cash, and some basic combat shit (rune) and yeaah,
I'm lvl 83 combat nothing special just a normal main.
I've got 99 fletching, theiving and cooking what 99 next?
Either
Strength 63
Mage 56
Fishing 77
Woodcutting 90
firemaking 60
^they are the stats atm.
I'd go for mage, it's quite quick not to mention it can be extremely helpful throughout the entire game.
It's definitely boring and instead of +$ it's -$ but well worth it.