Go ahead, Soulflayer-EU. AFAIK, the most populated server in WoW and also the server with the highest Horde:Alliance ratio (~1000:1)
4823 gold to level engi from 1 to 600 (6316 with 1 SoH) on Soulflayer Eu-Horde. Price is EU-average
9034 gold to level engi from 1 to 600 (13534 with 1 SoH) on Soulflayer Eu-Alliance - but supply is very limited.
For a rough estimation and assuming mining/alchemy are available, it would cost like 160k on a viable AH to produce a Sky Golem right now.