Eilid pays for her Animal Ken potion with 2p of Intellego vis.
Boudicca pays for her 3 Heals and 3 Restore Fatigues with 14 pawns of Corpus.
As it stands, Isobel has enough Corpus to brew some more Fatigue potions. But she has no Auram, so she can't brew any Flight spells. Unless someone can advance her some Auram, she'll probably brew more Fatigue potions for Spring 1221.
You do have a resident Pawnbroker. 
She doesn't need Flight potions. She can fly without them.
I don't understand.
You seem to be looking for Auram vis. It can be provided, whether via trade or loan. You lose a little in the process, but if you gain enough from it it could still be worthwhile.
It's just under standard Mercere rates of 2:1 Co:Au, Isobel would make no vis. She'd have to spend 14 Co to get 7 Au, and then she'd sell the potions for 14 vis, a wash. No sense wasting a season brewing when she can do other things. Besides, there's Au vis in the covenant. If people really want Flight potions they can get her Au vis to make them, without anyone having to play an exchange fee.
Isobel only cares about brewing Flight potions if people want them. And if people really want them, they'll find a way to get Isobel the Au vis she needs to brew them. That way she can keep all the profit and doesn't have to share it. 
That's assuming that she uses the Mercere to trade vis. If she were to trade with anyone else, they may go 2:1, they may go 1:1, they may go 2:3, or she may trade vis for something else entirely. It all depends on who she trades with, what that have, and what they want.
There is also the different loan rate.
Anyway, Beatrix certainly won't be buying flight potions; she's strong enough at Rego and Corpus that she can fly well already. Plus she would normally ride Günther anyway.
Isobel can take 3 Experience Points for this thread.
I'll put the 3 xp into Bargain.