Inscription is the newest profession in World of Warcraft. What's the cheapest way to level inscription in world of warcraft?
http://www.elsprofessions.com/inscription/leveling.html provides the best levelling guide for Inscription.
Regardless of which guide you follow, the cheapest way (when only looking at gold as a cost factor) is always going to be to take up Herbalism as well or have an alt with Herbalism.
Using your favourite Inscription levelling guide and Wowhead for example you can find the best routes for specific herbs and farm away.
Some addons such as Routes and Gatherer also help improve the speed in which you farm your herbs.
That doesn't mean it is the fastest but that would be the cheapest way as it costs you no gold.
However, there is an argument whereby time also is a sort of cost. Due to that, a lot of people find it more cost effective to do other tasks which take less time that make the gold you need to buy the required herbs for levelling Inscription with spare change and time left at the end.
But that is another whole topic in it's own I think.