There is a highly informative guide on the cost of leveling crafting we came across on the AionSource Forums. It estimates the cost of leveling a crafting skill from 1-399 using work orders, which is supposed to be the most inexpensive way. Here are some highlights of the guide:

• Each Crafing profession costs an estimated 2,000,000 (2 Million) Kinah to level up to 399
• Leveling all 6 Crafts to 399 will cost an estimated 13-14 Million Kinah
• 1-99 Lesser Skill Cost: ~3500 Kinah
• 100=199 Regular Skill Cost: ~18,000
• 200-299 Greater Skill Cost: ~120,000
• 300-399 Expert Skill Cost: ~480,000