I've spent the past hour of my life making a spreadsheet since I was thinking about picking up Jewelcrafting. Provided you have a spreadsheet program, and you're following WoW Jewelcrafting Guide 1-450 | Jewelcrafting Leveling Guide, you can use this spreadsheet as a guide to see how much it will cost you, and how much you will get back, from levelling Jewelcrafting (NB: Only levelling, not after hitting 450):
LtdGoldSS.xls
It assumes that you have an alchemist (or you know an alchemist) than can transmute gems to Skyflare Diamonds (You can substitute this with Earthsiege if Eternal Fire is cheaper than Eternal Air on your server) and an enchanter that can disenchant any item up to iLevel 125.
Assuming that you get 2 dust or 2 essence per dis-enchant (Considering the rate is 1-5 dust or 1-3 essence, this is a decent measure), on my server, Jewelcrafting would cost me a NET amount of 151g 73 silver. It would cost me 3,853g 54s to level enchanting, and I would gain 3701g 81s from selling dusts and gems from levelling. I did not include the green outland gems you can make because no-one will buy them. You'll make 5g if you sell them to a vendor.
You should ONLY change the Unit Cost column. The Quantity and Collected column on the first one was just for my purposes.
* PLEASE NOTE THAT THIS IS AN ESTIMATED PRICE AND IS SERVER AND FACTION DEPENDANT *
I hope this helps some of you.