Farming herbs (toadflax and above) has pretty much always been profitable.
For example toadflax seed is around 2k each, (each seed gives average of around 5 - 7 herbs, depends on your luck and farming level.)
Clean toadflax at the moment sells at about 3k each and you can plant all 3 - 4 herb patches in under 10 minutes. That is 40k / 52k gp per farm run if you get 5 herbs on average, with 7 herbs on average you make 57k / 76k per run depending on amount of patches.
However amount of herbs you get can vary a lot, sometimes you may get up to 40 herbs from 4 seeds and sometimes 10. Magic secateurs increases the yield so that might be worth obtaining before starting.
And the best part is you can make these herbs run every 2 hours (little less) or so taking only 10 minutes each of your time per run so it is worth it.
Also when you hit higher levels, grow some papayas / coconuts, whiteberries and cactus spines, do farm runs with those as well to increase your profits.
Also note: Always use supercompost to increase your yields!