Of course, the real money maker is saffron (as implied by YR7, on the first page of this discussion), which is made "merely" by plucking the stigma of the saffron plant. Which means that making a big pile of the stuff is a lvl 1 CrHe effect.
Depending on how you interpret the Size rules of Herbam, the base target is either a 1x1x1 pace bush, or else a 1x1x1 pace block of solid plant matter. Regardless, this is probably at least 100 pounds of plant matter in that. In finding this oddly-specific website (aqua-calc.com/calculate/food ... -to-weight), it looks like a cubic meter of saffron weighs in at 143 kilograms, so it looks like I'm in the ballpark.
According to the article, saffron cost 128 pennies/pound. So 143kg = ~314 pounds
128 pennies/pound * 314 pounds = 40,192 pennies, or just over 400 mp.
(EDIT - or I could just look at another table, where they tell me that it cost 400 pennies/kg in London in 1438)
Of course, creating only a cubic meter of saffron is a (base 1 + 1 touch + 4 permanent) = 10 effect... which is too low for a ritual. Add on 2 magnitudes for size, and you've got your lvl 20 ritual, and enough saffron to crash the European spice market. If you could sell it at market value, that would be 400 mp x 10 x 10, or 40,000 mp.
Certainly you wouldn't have to sell it all at once. Have some sort of Circle-based "keep my plant matter fresh" ReHe effect, and you can simply store the remainder, and sell a hundred kilos or so of the stuff every year (descretely, of course, to various spice merchants all over Europe), and fund your covenant like that.