I've got one PC who has more or less mastered the herbalism system. He's intensely anti-social, but does gather a large amount of herbs on his own, and very "quietly" pays ridiculous sums for herbs from other gatherers he's associated with. He's hardly shorted when it comes to herbs, but in terms of the healing variety there is a definite trend I've noticed.
Supplies to make moderate healing potions are never in short supply. If there was anyone just wanting moderate potions, any of the servers herbalists, even those of lower caliber (able to make a cure moderate without fail) could punch those out in large numbers. Cure serious are the killer, because, like Bad_Bud said back there, the rare element of those potions is only available about 1/4 (or less) of the in game year. Nor are they in large amounts, even then. My PC can produce enough Cure Serious potions each in-game year to keep himself mildly stocked up on them... he couldn't provide enough overall to supply anyone else. Thats not greed, thats just the production rate. And he has his own healing spells to fall back on, so for him, the few potions he produces are just fallbacks, not the sole means of healing.
If it were me, I'd go ahead and introduce a fourth healing herb element into the mix. I would make that herb available 1/2 of the year, and the combination of the 4 produce a cure critical potion. It would increase the output of healing potions of good caliber from herbalists and provide them a higher grade product to put out. Heck, jack the DC of a cure critical above and beyond the current max DCs, give the herbalists something new and harder to aspire too. Alternatively, fey leaves could be made more frequent or in higher quantites, that would pump up the output of healing potions as well.
These are just a few ideas toward the economy end of herbalism. As things are my herbalist can provide for himself in healing terms, just not enough to market and sell to anyone else.