When Patch 4.0.1 arrived in mid October 2010 glyphs went from an average 15 gold on my server to over 150 gold, and today i am selling glyphs for an average of 100 gold.
In my mind, there were five main reasons for this:
1. Announcement of the change of glyphs in the summer of 2010: over the summer Blizzard announced that the glyph system was to be changed. At that time, glyphs were destroyed when they were swapped out – hence characters had to buy a new glyph every time they swapped one out. When the change was announced to the current system many forecast the death of glyphs. Hence, there were very few new competitors and existing competitors started to leave the market. The view was that glyph prices would collapse. Hence, when the patch arrived the number of scribes was low and indeed the scribes left had reduced their stock of glyphs.
2. Demand went through the roof and the supply was not there: When the new glyph system arrived characters went to the AH to buy up a complete set of glyphs on the first day. What glyphs were on the AH ran out very quickly. Indeed, i could not post quick enough and hence the selling prices rose rapidly.
3. Players returned to the game: on Patch 4.0.1 and generally though the Cataclysm launches old players returned and so added to the demand for the glyphs. Indeed, there were various surges of demand as new patches came and old players came back into the game.
4. Inks from 1 to 3 per glyph in October 2010: the cost of crafting tripled but more importantly the demand for herbs rose firstly due to the tripling of materials required per glyph and secondly due to the strong rise in demand. Hence, herbs started to run low and their prices rose forcing the prices of glyphs higher still.
5. Warden in summer of 2010: in the summer of 2010 Blizzard launched a program to catch and ban bots. It was very successful and overnight many bots left the game. However, their herbs remained on the AH and in their guild banks (i assume the AH posters were on different accounts to the bots though i guess Blizzard can now detect that?). Hence, whilst the bots were gone, the herbs were still hitting the AH at very cheap prices. The timing of those cheap herbs running out varied by server. On my server, it happened just at the time of Patch 4.0.1. Hence, the ongoing availability of herbs went through the floor which was a third factor pushing up the price of herbs and restricting supply to scribes trying to meet demand. Indeed, for myself, i was often in danger of being unable to meet demand on several occasions due to lack of herbs to mill.
In summary, demand rose suddenly at a time when supply was low and availability of raw materials was low.
for this and other ideas on the glyph market please follow the link to the free Croda's Inscription Gold Guide