Farming not profitable in higher tier?

Her guys, i was just checking on different methods to make silver...remembered i have a island with a bunch of farming plots....why is farming T5 dragon teasel sell for almost 2x than the 2 tier higher T7 Mullein? Am I missing something? Is it just better to not progress in tiers when it comes to herbs and stick with T5?

Let's assume I got the IQ of a mildly above average toddler....also I'm a returning player who only really played conservatively here and there.