Also, the cheapest way to improve an visio setup is a correct audio installation, with a good enough mic and a soundcard. ~200€ gives you something correct, and is a huge step-up when doing visio (got a entry/mid level mic + id4 Audient soundcard, it does wonders).
For people starting from computer cam + headphones, I'd recommend upgrading audio first (cheapest, biggest felt quality improvements for others), then upgrading the video setup as explained here.
Having said that, there's no arguing that a better sensor and a wider aperture lens that gives a shallower depth of field yields more pleasing results.
The problem is, I hardly know anyone who's willing to pay almost $1000 to achieve such results.
If I'm recording a "proper" video, I need the higher quality that comes after pressing "record", and I jerry-rig a computer fan onto it. At webcam resolution I don't notice enough difference to worry about it, though.
That said, this could well depend on where one's heat source (e.g. windows and sunshine, radiators, ect.) are. Mine happens to be well away from anything else that generates much heat.