So far, the data I've been gathering for a while suggests that on average, you get 7.6 herbs from My Arm's patch, and 6.6 herbs from other patches (including deaths). Just counting GP, the best herbs to grow are Torstol in My Arm's patch and Avantoe > Ranarr > Toadflax in the other patches (as of post time). Including cleaning xp, Torstol is still best for My Arm's, and Avantoe > Ranarr > Snap > Lant > Toadflax for the other patches.