Prices for trainable recipes will quickly settle just above the cost of making the item, because almost everyone will be able to make these items. For low-level glyphs, that could be a couple of silver: A glyph like Backstab averages around 3-4 Peacebloom, plus a 20 copper for the parchment. Higher-level glyphs will cost a few gold (about 7 Felweed, plus 50 silver for the parchment, for something like Glyph of Mind Flay). Northrend glyphs may exceed 10g each.
(There's currently an issue with the vendor prices for glyphs: They're all about 38 silver. If the current code goes live, any herb stack sold for less than about 2 gold should be purchased, turned into glyphs, and vendored.)
Discoverable major glyphs will be fairly rare for a few weeks, and will allow Inscribers to charge a premium. Northrend research is a skill 385 recipe, with only a chance of discovering something. Minor discoverable glyphs should be rare (it takes 2 months to find them all), but will not be, because minor discoveries can be made by level 5 alts - there are lots of those about.
(That's the second money-making reason to roll Inscription on your low-level alts...)
I originally thought people would sell pigments and not inks. But now the recipes have been simplified, so that (with 1 exception) each ink uses a unique pigment, ink-selling may occur instead.
Overall, Inscription is a fundamentally low-cost, high volume profession, very similar to Alchemy. That's why we're so concerned that it doesn't have a clear, highly used consumable yet...