I know a very inexpensive seller on ebay. You can get the full setup, 400wHPS ballast, reflector, 400wHPS bulb for $100.00, you can get an additional 400wMH conversion bulb for the same ballast for $125.00
Let me know if your interested.
But it doesn't sound like you would have the room. For a very limited space, CFLs are definitely the way to go.

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