Well here’s the thing, I don’t think it's possible to get just the case(unless you have a place that recycles old laptops near you and they are willing to sell just the case). You actually have to buy a barebone system, which can usually be picked up for $200-500. If your willing hunt around I’m sure some store sells used or refurbished laptops for under $200. They are usually as slow as molasses but since you’ll be ripping the majority of the hardware out to put in new and shiny parts I don’t think it really matters
