1.) When did magazines get termed "clips".? Everone (inteligent) knows the difference, but have you ever heard of a Banana magazine?

2.) Will 30 round followers work in 40 round magazines?

3.) Why the hell are 20 round magazines more expensive than 30 round magazines? They contain less material and one would assume (or at least I do...) that they are less sought after... What gives?

Here are a few random answers, yes, no and no.
But, seriously,
1a) The use of the word "clips" is a carryover from times when the early semi-autos had internal magazines that were reloaded with stripper clips.  After that it was, just as you indicated, a lack of knowledge.
1b) The term "Banana magazine" is used when the magazine curves like a "banana".  Most visible on magazines used where the rim and/or rear case diameter is larger (usually much larger) than the shoulder of the case and the ammo stack forces the bottom of the mag to curve forward as can be best seen on AKM/-47 magazines and most of the old .303 British mag fed Bren guns and those of several other countries.
2)Yes they will as it is usually work. It is the springs that are not compatible.  But, with the exception of some of the newer >30 round AR mags by "name" mfgs, the mags are not reliable and pose other issues.
3) Most "good" 20 round mags are USGI and are not being manufactured in quantities these days of the 30 rounder dominance.  Market availablility still drives price.

1b) The term "Banana magazine" is used when the magazine curves like a "banana". Most visible on magazines used where the rim and/or rear case diameter is larger (usually much larger) than the shoulder of the case and the ammo stack forces the bottom of the mag to curve forward as can be best seen on AKM/-47 magazines and most of the old .303 British mag fed Bren guns and those of several other countries.

Yeah, i ynderstand where the banana part came from. I was just trying to exemplify the dominance of the word "clip", which is why I italicized "magazine". Hollywood, the media, and the uninformed bandy about the term "Banana Clip" like it's going out of style, but I have never heard anyone ese "Banana Magazine"...

Oh, and thanks for your answers.

Personally, I would like to get a few 20's, but I'm not gunna pay more for less capacity.  D&H makes a 20, that you can get for about 15 bucks.  

Stay away from 40's.  I haven't ever seen a mass praise of them, or much praise eithor.  Stay with USGi mags, they are cheap right now, and the most standardized mag avaliable (which translates into a better deal and better quality...if you get GOOD mags).  

The term clip/magazine probably comes from movies (probably).  I don't know why (maybe more gangsta' to say clip vs. magazine).  Hell, I dunno, but it bugs me when supposedly knowledgable firearm owners refer to a mag as a clip.  We know better/at least we should.  (unless you are using a clip)

I agree. All my mags are 30 rounders except one 40 and it sucks. The follower tilts and jams every now and then, which is why i asked about followers. But, yeah, I also agree about paying more for less. Although I want more variety in my accessories, economics says "no".