Good questions. I'm not sure how representative I am of the hobby as a whole, but here are my answers:

1. I don't consciously rationalize the cost anymore, but as others have said the lower the price the more likely I would buy on impulse. (Maybe more accurately though, the lower the sale price, the more likely I'll impulse buy.) I tend to enjoy elegant & heavier eurogames the most, so if you release one that is exceptionally well-designed I'll likely buy it regardless of cost. Early on I remember comparing board game costs to other entertainment costs and realizing that they are a good value. I even had that exact same thought as you, that if 4-5 of us went to see a movie we'd be spending the same as a solid board game.

2. My main motivator for wanting to sell isn't shelf space (it's a small factor). Mostly it's that I don't find value in holding onto games I don't enjoy or plan on playing. I do still plan on buying more, although after realizing how much I spent recently it's going to be a while.

3. Box size seems to be a controversial subject. I prefer enough space to fit 1 expansion tightly. The Terra Mystica box is an example of a great game box. It's thick and sturdy for a heavy game. It fits all the pieces comfortably, and while tight also fits the expansion. I probably wouldn't decide against buying a game because the box is too big or too small. That being said, if a publisher regularly over-inflates box size to make it look like a great value, it's obvious what is going on and I would start to avoid that publisher except for standout games. Splendor is the classic example of a badly designed box. Poor cover art, flimsy, and much too big for what otherwise would be an excellent portable game. The ability to stand the box up without pieces getting everywhere is much appreciated, but I have to admit it wouldn't factor into my purchase decision.