Look in car/truck boneyards for used boxes, they can go VERY cheap.
The side box on my truck was $30, two new locks (keyed alike) $20, Rustoleum and sandpaper $15. Total $65.
I've seen the nice aluminum side boxes go used for $50 each.
These are the sit-on-the-side-rail boxes, I've also seen those underbody boxes too. Keep a regular eye on your local junk and scrap metal yards, try Craigslist.org (find one in your area), or Freecycle.org (also one in your area).
Seen two pop-up trailers going for free hereabouts this week. (Boston area)