Are you aware that each Obsidian Gate gives an extra trade route? So a civ with 20 cities could easily gain 50 or more commerce from the Nexus, far more than 2 Obsidian Gates alone. It is a major economic benefit even if the strategic ability to move troops unlimited distances in one turn between cities is not used.
No, I was not aware of that.
20 cities is a hell of a nation. I would think that the NPC civs would be more worried about the massive armies that are about to come from your nation than the jump of your total income when you succeed at finishing a wonder.
I would also point out that you get a bigger cash bonus from the high temple buildings, and you get that wonder without researching any extra techs or doing any building (although you do need a Great Prophet).
But, at a cost of only 600 hammers, yeah... the Nexus is comparable to other wonders, but probably still too cheap.
Code:
Acuae Sucellus 600 Bone Palace 700 Catacomb Libralus 600
Celestial Compass 500 City of a Thousand Slums 500 Crown of Akharien 900
Form of the Titan 500 Grand Menagerie 120 Great Library 350
Great Lighthouse 600 Guild of Hammers 550 Guild of the Nine 500
Hall of Kings 600 Heron Throne 250 Infernal Grimoire 400
Mercurian Gate 600 Mines of Gal'Dur 700 Mokka's Cauldron 600
Pact of the Nilhorn 500 Pillar of Chains 500 Prophecy of Ragnarok 500
Ride of the Nine Kings 600 Shrine of Sirona 500 Temple of Temporance 650
The Eyes and Ears Network 1,500 The Nexus 600 Theatre of Dreams 500
Tower of Complacency 750 Tower of Eyes 700
Short examination here:
Tower of Complacency (No unhappiness anywhere) [750]
Theater of Dreams and Temple of Temporance (+1 happy everywhere) [500 plus three Theaters, 650]
Infernal Grimoire and Eyes and Ears Network (One free tech, or about four lower tier free techs) [400, 1,500]
Bazaar of Mammon (+100% gold, at least 50 coins, probably more) [600 and most expensive non-tower National Wonder in list]