I buy old kits that I had and built as a teenager many moons ago. My mother threw them all out when I went to college. Having built them before, I know what kits were sweet and which were duds, so I buy only the ones that were important to me at that time. I often buy two samples. One that is mint in the box, and which I intend to keep that way, since I have already experienced the "build", and another identical kit that is opened, or sometimes started (you can always strip the paint off) for really cheap which will become a new "builder". Other models that I know are OOP I will still build if it adds to the theme of my collection, even if I know that it has a great value MIB.
Which leads me to wonder: Anyone can shrink wrap a box. How do I really know if that $100 IMC Ford GT 40 MISB on my shelf doesn't really contain a bunch of old stripped trees? Am I stupid? Maybe. Can I afford that? Yes. Does it matter? You have to believe in some things. The only blessing is that I have two others in opened boxes that I got for less than $15.00 total, so at least I will have a built (by my hands) example that will be worth more than money to my children and grandchildren. So, I guess the bottom line on the value of kits, in any condition, is all in the mind. Build 'em if you like 'em, but build them with your heart. If it shows that it can be an investment (My old Renwal Visible Wasp Engine goes for around $500, another of Mom's throw-outs) then invest. Sometimes the twain doth meet.