$100 Jace is literally the only reason I played in a series of local T2 events recently. I had almost all of the expense covered, so getting some other UB stuff was a non-issue. I have also traded a ton of Jaces to people looking to acquire them. They're not hard to find at all, they're just valuable, so you have to shell out to get them.
For those who are against $100 Standard rares, what is the reasoning? It seems random to put an arbitrary cap on what other people should be willing to spend on cards. Do people believe $100 Standard rares are preventing Magic from being something they'd want it to be? Do people against these prices have issue with Golf costing as much as it does to play?
I look at $100 cards in Standard as a sign that people actually want them, and Wizards has not flooded the market on those cards. If people are willing to spend $100 for a Standard card, there is a huge demand for the game and it's parts. I see this as a self regulating system, just like cards on the Reserve list. If the cards are worth a lot, that's because people are willing to spend money on them. People wanting Magic cards is a good thing for the game of Magic. If the price gets too high for some people, they'll sell them to people who want them more. If everyone is overvaluing their cards, and no one is willing to buy at the asking price, there will be a market correction. The price will go down until the market begins moving cards again.
Although earlier I mentioned Elspeth Knight-Errant and Baneslayer Angel as cases where reprints lowered their price to more tenable levels, I'm actually going to say the reverse now: part of the reason people don't like $100 standard mythics is 'planned obsolescence'. Needing a $100 card, or playset of such cards, is one thing, having that investment disappear when the card is reprinted, rotated, or truly obsoleted by power creep is another. Especially when the cycle starts over with some super-powerful mythic from the most recently released set. Golf clubs (or in my experience, skis) are much more akin to durable goods, and don't suffer from the same planned obsolescence problem as a game piece with artificially limited legality.
Eric is 100% right that prices are being set by the interaction of supply versus demand, but I don't want to feel priced out of Legacy (or 0-proxy Vintage) because I'm not willing to pay $100 per JTMS while knowing it's going to be worth what, ~$45 after rotating out of Standard? It's not economically efficient for me to invest in a set of Jaces if I'm not going to compete in the more lucrative Standard competition. Again, all of this is compounded, in my opinion, by what I said above about abolishing Mythic rarity: mistakes and hyperefficient cards excused by their rarity alone.
$100 for a Type 2 card seems ridiculous to me, but $200 for a Candelabra of Tawnos when it was $30 6 months ago seems ridiculous too. Life can be ridiculous.
There are 31,000 Candelabras, minus those lost/destroyed. It's used as a four-of in the deck it's played in. Compared to Tabernacle's rise from ~$60 to ~$300 after seeing marginal success as a 1-2 of a while ago, this isn't unbelievable to me. I wish I could find the one I own.

Finally, I just want to state that Jace should not be anywhere near $100, because it should have been banned in Standard already. It is clearly dominant and warping the metagame. Its presence in the varies, but I've seen a number of events where it was putting up Skullclamp-like numbers in the meta, and I would expect Jace to get axed in some format or multiple formats sooner or later.
I doubt wizards is going to ban a planeswalker... ever. They're flagship marketing cards. As a non-standard player I'd have been all for this, though at this point I plan to just wait for rotation. Also, Jund was pretty damn dominant ~1 year ago, with 24+ copies of Bloodbraid Elf in every top 8 for months on end, and didn't see any bans. Bloodbraid Elf was an uncommon though.
The brutal downfall of BSA can also be attributed to her Majesty being relegated to fun casual card in face of the 6/6 monstruosities that ruled over Standard. If Action includes a third version of Jace with the same CMC but 5 abilities including a +1 Loyalty Ancestral Recall instead of a 0 Brainstorm, I'm sure the price of JTMS will fall. I'm not sure this will be a good thing for MtG, though.
You're right about that, and I agree that power creep (hyperefficiency was the word I used above) is not a good thing for MTG. It's downright embarrassing that a 5/5 for five
with five positive abilities is fringe playable in the smallest format it's legal in. Note that creatures that don't generate card advantage immediately upon entering play, like Baneslayer, also suffer from Jace's prevalence, since bouncing a single creature once resolved generally leads to gained value.