I used to do slayer + farming.
I'd farm herbs in all 5 patches and slay while they grow.
Farming alone isnt amazing money but it soon stacks up next to other tasks. You get money while asleep and before you notice it you're 90+ farming.
It isnt as easy as "make the best herb you can" you need to look at the price per leaf. the average yield per seed (6.8 with secatures and super compost i believe)
and also the loss rate on the seed + the seed price to get the best GP/H one.
Either way, its a good little side earner that makes you money while you do your normal tasks/slaying/whateveryoudo
and it racks in good farming exp which is a difficult skill to "grind" and to get 99 farming fast costs a LOT of money on magic seeds, palms etc. If you can do the bulk of 99 while making money.. why not!