Lets get started!
Step 1 - Work out your Blackfallows to Inferno ratio. This figure is basically, how many blackfallows is one inferno worth to you? Personally I set this as 10 because infernos convert into DM trinkets which sell highly well but everyone is different. For example Foo, at time of his comment, would use 4.5.
Step 2 - Use Auctionator and search for Whiptails. Focus only on auctions that are full stacks (20) or at least multiples of 5. Ignore all other stacks with funny sizes including singles. Look at the single item price (circled in red). For this example lets use 1.6g.
Step 3 - Look at the Whiptail pricing row and find 1.6g. Raw material cost is 6.5g (labor not factored in).
Step 4 - Decide the max Glyph Cost price you want to work with. Lets say for example you are ok with 8g per glyph. Go back to Auctionator and snatch (buy all quickly) all the full stacks of herb below that price.
And there you have it, that's how you use Critical's secret secret costing calculator! Sorry if you were expecting something super fantastically amazing. Critical doesnt do amazing =/.