It's a print-to-demand set, the prices will probably stabilize somewhere sane in a month or two.
On topic, I'm not sure what to think about Urza. I see the appeal, but I don't think he's necessarily better than Whirler Rogue. The two Thopters are going to better than the Construct on average; the unblockability is probably worse than the ramp on average, but it's a situationally great effect; and the Mind's Desire is probably going to be disappointing. I might try him out, but I'm really not sold.
I like your rankings except for Expanse and Wilds. The land entering tapped is a really big deal. I think they make great budget options, but I would put it after city of brass. I have never played with paradise and gemstone so I can't comment on those.
The pain from City/Confluence adds up to a real limitation in a significant number of games, but you're probably right. Conveniently for me, the top 6 (post-MH) are pretty solidly above the rest.
I would suggest deciding how many 5-color fixing lands you want in your cube before you think too hard about which ones you run. If you're deciding between and individual artifact and an individual fixing land, I think you're coming at it from the wrong angle.
Green seems worse than Chord of Calling, +2 mana without convoke hurts. I still might give it a try over Zenith, not sure. But I'm not optimistic.
White is really solid. It's okay at 4 mana and very good at 5 or more.
Blue is just bad.
Red is probably too narrow, and loses the most by being sorcery speed.
Black is the most interesting. If you don't have a board, it's way worse than existing sweepers. But if you're playing something like midrange vs aggro, it can be exactly what you need to stabilize. I think that still ends up being worse than other options.
Getting a 2/2 and a (delayed) card on the [+1] is a pretty big deal, I think, and the removal at [-3] is probably better than Ob Nix or Teferi's. The passive curves nicely into old Ugin and other curve-toppers. The low loyalty if you have to start with removal does hurt the card. Overall the card seems pretty good.
The incidental lifegain is pretty tempting to help control decks stabilize from a low life total, and the body+removal is nice. Dimir is deep, but I could see this getting there for larger guild sections like mine.
I'll probably want to include a few more haste sources to support this guy, which may be something I should have been doing anyway. I think having the ability on a 6/6 trampler for 5 is just great.
On topic, I'm not sure what to think about Urza. I see the appeal, but I don't think he's necessarily better than Whirler Rogue. The two Thopters are going to better than the Construct on average; the unblockability is probably worse than the ramp on average, but it's a situationally great effect; and the Mind's Desire is probably going to be disappointing. I might try him out, but I'm really not sold.
Among the lands, I would rank them:
Prismatic View
Ash Barrens
Terramorphic Expanse / Evolving Wilds
Mana Confluence
City of Brass
Undiscovered Paradise
Gemstone Mine
I run 6, so Undiscovered Paradise gets bumped.
White is really solid. It's okay at 4 mana and very good at 5 or more.
Blue is just bad.
Red is probably too narrow, and loses the most by being sorcery speed.
Black is the most interesting. If you don't have a board, it's way worse than existing sweepers. But if you're playing something like midrange vs aggro, it can be exactly what you need to stabilize. I think that still ends up being worse than other options.