Inscription by far cheapest profession to lvl. You most likely need around 2k for plants (+/- 1k) to lvl it. If you check right glyphs and sell them on AH you propably end up making profit. I actually made around 3k profit when i lvled it.
Another easy one is tailoring, only northrend cloths may bring slight problems. If you farm these yourself you will lvl very cheaply.
And also alchemy (another alt was around 3k to lvl to 600) is piss easy.
Server dependent question, but I'd say the following, from cheapest to most expensive (without farming)
1 - inscription
2 - tailoring
3 - jewelcrafting
4 - alchemy
5 - engineering
6 - leatherworking
7 - enchanting (this can be lower if you level a crafting profession along side it, or haven't done much questing)
8 - blacksmithing. The range between fel iron bars and saronite bars can be more expensive than 1-600 in other professions.