You've hit on two of the reasons I stopped being a completist here @Likefro - stuff that's too expensive and stuff I didn't care about.
For some reason that I can't quite recall, I decided many moons back to go for every American Spider-Man comic and appearance. And I mean every one. Maybe @1950's war comics is right on the OCD claim earlier, but I really went for it. Look at my old records - these are just the title indexes:
At its peak, I think I had over 8,000 comics with Spidey in. Every main title. Every mini-series. Every one-shot. Every promo. Every variant. Every printing. Every appearance. And I mean every appearance - one panel was enough to warrant getting the book. Strange Tales 97 because the Parkers looked like they were in there. Spidey on DD #1 cover? Got it. I spent £500 on a comic just because he was on the cover.
Looking back, it was madness. But my God did I enjoy it. Up until the point that is when I almost got everything. The moment I'd 'done it', the full magnitude hit me. 8K comics, of which about 6K were, for want of a better word, tat.
And then the variant madness went up a notch. I purchased all the 666's, all the V3 #1s. And then one day I stopped. Dead. I went cold turkey. And then I sold everything except for 50 or so books.
I'm keeping this brief and could write a thesis on it. Maybe one day I will. So beware, completists. It's a dark road to go down unless you are fabulously rich, have all the time in the world and plenty of space.
Alas, I no longer have those last three...