Right now many late bronze Uncanny X-men keys can be found for a good deal. look at what happened when the hint of a Kitty Pryde film started recently. X-men 129 practically doubled in 9.8 and tripled raw. This was an already popular book given it's also Emma Frosts first appearance. these examples could be called copper but first few fall right in the end of bronze age.
X-men 160 1st appearance of Magik still a few bucks per, amazing story and art
166 1st Lockheed
168 1st Madelyn Prior
169 1st Morlocks, Callisto
184 1st Forge
201 I still find these in dollar bins at Eides and New Dimension
205, 207,
210-213 -mutant massacre,marauders, Sabertooth Vs X-men classic!
221,222 1st Sinister Sabertooth Marauders vs X-men
I suppose the ship has sailed on a few of these such as 221 but I'm surprised how I can still find these in VF+ from time to time in $5-10 bins.
However the true Gold is in anything Hope Summers right now for quick flips, as we've already seen Cable with the baby gear (teddy,bottle)
X-men 205 1st Hope Summers
New X-men 128 1st Fantomex
of course this a bronze thread so..
Fantastic Four #211 1st Terrax.. this guy used to be way more relavent in the 80's and early 90's being a herald and all.. I could see him being a mainstay venom like anti hero once the FF get rebooted, Terrax is a good way to bridge MCU. much cooler character than Korvac or the countless space 'gods'.
Whats great about the list I made is that the stories are actually good. Investments worth reading..