how come Jace the Mind Sculptor is worth so much?

[rocky]

why is Jace the Mind Sculptor so expensive?

[Arix]

He costs that much, frankly, because people are willing to pay that much.

[rocky]

why are people willing to pay so much money for him he costs 10 weeks worth of pocket money

[Kenji]

10 weeks? I suppose that's a matter of how much money you make. I could buy a Jace within a month's worth of disposable income, but I'd really rather not as I'd consider that wasteful for me.

Anyway, the reason why people are willing to spend that much is because Jace, the Mind Sculptor is incredibly good, to the point where every single blue-based control deck (in Standard, anyway) is made better by having 3-4 copies of him.

Hence, this is an issue of supply and demand. The supply is low (due to Worldwake being a fairly unpopular set, outside of Jace, which means not as many packs were opened of this set), and demand is high (because almost every blue player that wants to win high-level tournaments needs multiple copies).

TL;DR -- Economics is the reason why JtMS costs so much.

[Arix]

Also consider that for professional players who play for big money in pro tournaments do everything they can to win (and with huge cash prizes up for grabs, whywouldn't they?), spending that much on a single card to greatly improve their chances of winning said prizes could be a sound investment.

Personally, I think that despite all these reasons it's still a completely ridiculous price, but hey, I've got three, so I'm not complaining.