Since Wrath of the Lich King went live the prices of herbs have increased by a lot. If you're a little poor on the gold side you should consider Herbalism as your companion profession. With herbs frequently going for 20-40 gold (or more) per stack you will save a ton of gold by gathering your own.

Alchemy
The alchemist mixes herbs, gathered by Herbalism, and reagents in order to generate elixirs, potions, oils, flasks and cauldrons with a variety of effects. A player can create healing, invisibility, elemental resistance and mana potions; oils to coat weapons; and much more.

Alchemy is useful to all character classes. Melee fighters can use alchemy to create buff and healing potions. Spell casting classes can use alchemy to create mana potions and related buffs. You can create potions for yourself, sell them to others, or give them to your party members and friends. Alchemists are well loved when they hand out potions.

With the price increase in herbs since Wrath was released it's hard for Alchemists to make a profit, though some potions will sell well. Check prices on your server and eith gather your own herbs (to keep costs down) or keep an eye on the AH for screaming deals on herbs.

1-155
1 - 60
Minor Healing Potions (1 x Peacebloom, 1 x Sliverleaf, 1 x Empty Vial) x 60
60 - 110
Lesser Healing Potion (1 x Minor Healing Potion, 1 x Briarthorn) x 50
110 - 140
Healing Potion (1 x Bruiseweed, 1 x Briarthorn, 1 x Leaded Vial) x 30
140 - 155
Lesser Mana Potion (1 x Mageroyal, 1 x Stranglekelp, 1 x Empty Vial) x 15

155-250
155 - 185
Greater Healing Potion (1 x Liferoot, 1 x Kingsblood, 1 x Leaded Vial) x 30
185 - 210
Elixir of Agility (1 x Stranglekelp, 1 x Goldthorn, 1 x Leaded Vial) x 25
210 - 215
Elixir of Greater Defence (1 x Steelbloom, 1 x Goldthorn, 1 x Leaded Vial) x 5
215 - 230
Superior Healing Potion (1 x Sungrass, 1 x Khadgar's Whisker, 1 x Crystal Vial) x 15
230 - 250
Elixir of Detect Undead (1 x Arthas' Tears, 1 x Crystal Vial) x 20
250 - 300
250 - 265
Elixir of Greater Agility (1 x Sungrass, 1 x Goldthorn, 1 x Crystal Vial) x 15
265 - 285
Superior Mana Potion (2 x Sungrass, 2 x Blindweed, 1 x Crystal Vial) x 20
285 - 300
Major Healing Potion (2 x Golden Sansam, 1 x Mountain Silversage, 1 x Crystal vial) x 20

300-375
300 - 315
Volatile Healing Potion (1 x Golden Sansam, 1 x Felweed, 1 x Imbued Vial) x 20
315 - 330
Sneaking Potion (2 x Ragveil, 1 x Felweed, 1 x Imbued Vial) x 20
330 - 350
Super Healing Potion (2 x Netherbloom, 1 x Felweed, 1 x Imbued Vial) x 20
350 - 375
Major Dreamless Sleep Potion (1 x Dreaming Glory, 1 x Nightmare Vine, 1 x Imbued Vial) x 40

Beyond 375
You will need to have the Wrath of the Lich King expansion and you will need to head off to Northrend to get your Grandmaster alchemy training.

Overall Estimated Required Materials:
20 x Arthas' Tears
40 x Blindweed
80 x Briarthorn
30 x Bruiseweed
90 x Crystal Vial
40 x Dreaming Glory
75 x Empty Vial
60 x Felweed
60 x Golden Sansam
45 x Goldthorn
100 x Imbued Vial
15 x Khadgar's Whisker
30 x Kingsblood
65 x Leaded Vial
30 x Liferoot
15 x Mageroyal
40 x Mountain Silversage
40 x Netherbloom
40 x Nightmare Vine
60 x Peacebloom
40 x Ragveil
60 x Silverleaf
5 x Steelbloom
40 x Stranglekelp
70 x Sungrass
Complete Alchemy Leveling Guide
This is a compact overview to increase your Alchemy skill to the level 70 skill cap. For a far more in-depth guide to Alchemy, plus all the other WoW professions, including gold-making opportunities, please take a look at Penn's WoW profession guide.