First off check craigslist.com, I often see growers closing shop and selling their equpiment for a good rice on there.
Check in the yellow pages for an electrical supplier/ wholsaler in your area they should have HPS/MH systems for way cheaper than any hydro shop.
Or you can make your own, there may be a guide in the growfaqs here, but if you check around the other stoner sites you will find instructions on how to make your own, it's a good way to save a buck.

And as for 400 watts over a 5x5 area, they are talking about using it on common vegetables, and fruits, not Mary Jane.
peace