There's no trade subforum because the site owner does not allow for trade or sales topics on here. It is explained in VOGONS is no marketplace! but not listed on the site's rules so there's always some confusion about this.

Also not trying to be a smartass or anything but those cases/systems are that expensive because they are sort of iconic and sought after and there isn't an openly accessible ample supply of cheap ones anywhere; the whole supply and demand thing. On enthusiast forums people know the going rates for those items so scoring a deal on such sites isn't really likely.
You will probably need to do what most of us users without large amounts of disposable income do and keep your eyes open browsing local classifieds, yard sales, fleamarkets, ebay, facebook market place and the likes hoping to find a good deal if you're looking for a rare item for a reasonable price.
Those things do show up if you keep your eyes open. It just takes more patience than buying new parts. Good luck! 😃👍

I see several XT clones on Ebay right now (claimed to be tested/working) for under $250 shipped. I purchased mine from one of the large sellers (over 6800 feedback 99.9%) for really cheap since he accepts reasonable offers up to 25% off, and one time he accepted 50% off from one of my offers.

If you're looking for an actual IBM, good luck. Most people know that collectors will pay a lot for them.