You want cheap ThinkPad, you best stop thinking about series.
Cheap ThinkPads (and most of second hand laptops) almost always come from decommissioned laptops after the company upgrades its fleet (Note that, decommission doesn’t mean “not usable”, in fact, it proves the laptops can stand a test of time)
Looking for a specific series of ThinkPad might spike up the price.
Just remember to change the batteries. You’ll be Golden!