Lets take a grey life potion, this lasts all the time for each Avalon session.
If you bought this from a loremaster with Leg elixirs say 35 sips, assuming you supplied the bottle the cost would be:
200gp + 10 x 35gp = 550
As this potion would last you 35 games of Avalon that is 15.7 gp per sip.
Whilst all potions do not last a whole session the cost of an individual sip is probably in most case not much more than the equivalent herb (Kathuska/Brown Potion)
Also If I sell a bottle althought the cost is slightly more it will come with a grip rune. +e