#1
I know it determines how many pieces of wood are used in the building of the neck but which of them are cheaper, more expensive, harder to make, eaiser to make, better, worse.
#2
I would say the main difference is the number of pieces of wood....yep...I'm pretty sure that's what it is.

Actually the more pieces you have, the stronger your neck is, especially when you get different types of wood (eg. Maple n' walnut).

On the other hand, this may sound contradictory, but necks with fewer pieces resonate better and give more sustain. Same applies to guitars with one-piece bodies. Glue somehow mutes the vibration, although it's not that big a difference, really.

using smaller pieces saves wood, cos it's best to glue some small ones together to get it just the right size rather than get a huge chunk and waste half of it.
#3
One pieces are used in higher quality (and more expensive) guitars, seeing as they are more expensive to mass-produce. They're also a lot better in terms of sustain, like sashki said.