Make yourself a list of all the things you need/should have and rate each item according to necessity, along with your options for each one and how much it will probably cost. You can add the expenses up and make better determinations according to your budget.
For example: you're really going to need a complete lighting system, which will include the ballast, reflector, socket/cord set, and a bulb. I've found that some of the complete lighting packages available online actually end up costing a bit more than if you had selected each component individually, and the complete/'build-your-own' packages also limit you in terms of what reflectors you're allowed to pick.
I got a 400 watt Lumatek digital ballast and everything else from cheaphydroponics.com. Instead of selecting the complete lighting package, which would have cost me $270 with the cheapest air cooled reflector I'd have been allowed to pick, I got the 400w Lumatek, cord set, Plantmax bulb and a Sunleaves Aurora 6. I saved $15 by going with the Aurora 6 and an additional $15 for selecting each item individually; $239 for everything before shipping.
This might not be the case with every merchant, like that system darkdestruction420 linked to which is probably about as cheap as it gets for that kind of light but it doesn't hurt to break everything down and see if you can save. Shipping charges are also often ridiculous, so either get whatever you can from one place or maybe there is a grow store in your area. I got my light online because it was considerably less expensive than if I had bought it from the grow store that I typically get all my other stuff from, but I was unaware that an HTG Supply store was actually in my area until recently.