How good are DK's money-wise? My current stats are 99 att 86 str 90 def 91 hp. Also 87 mage and 78 range. So, how much money would I be making and what is the best setup and could I solo it?

You could probably solo mage Rex but im not sure if u can Hybrid or Tribrid though. When i did it i used Full Guthan's, Void Mace, Unholy Book, Guthix Cape (MA), Infi boots, Barrows gloves, Seers ring. Reason for the guthix cape is i was using Claws of Guthix with charge spell to do as much damage while solo maging rex as possible. Dk's can be an upwards of 3m from what i've been told although i normally dont top around 2m -2.5m if im lucky.

I am going to say your best off maging Rex, seeing as your range is a little low.

Alright, and would it be better to solo or duo for my stats?

If u have done DK's before solo should be fine, otherwise i would make a few trips with a friend or coach to show u the ropes because if ur not prepared for them it could be a very instantanious death...and bye bye cashpile.

You might be able to tribrid if you only use a cannon on prime with a 400-500 people server, but if you don't know what to do there, it will be very hard, plus, DK's are one of the monsters made spectacular by overloads(need to use all combat classes, heal with brews) and yak(bank hides and bones), so not having those will cut 4m profit an hour into something like 1,5m an hour.

You would need ovls/soul split to tribrid effectively.