3500 goods (20 gems) can turn into 12726 money with the corporate towers.
For 20 gems you can also buy 10000 money.
Turning goods into money also takes energy clicks. Turning 3500 goods into 12726 money with Corporate Towers uses 32 clicks.
Clicks are also worth money, since you can always use them to get 90 money out of a 30 minute Condo Highrise. 32 clicks is worth 32x90=2880.
So if you have no other buildings just Corporate Towers then buying goods is better.
But if you have 10 condo highrises then you can just buy 10000 money for 20 gems and use 32 clicks to get another 2880 money for a total of 12880. That's more than the 12726 money.
If you have Idea Factories as well and a lot of Glass Towers, then a click is worth 200 money. 32x200=6400. With 10000 at 20 gems that's 16400 for 32 clicks and 20 gems. More than buying goods and using Corporate Towers.
In short, buying money is better than buying goods unless you have almost no buildings. Buying energy is indeed a waste.
You're best off just buying awesome gem-only or expensive buildings though
Have a look at these gem shops:
Haunted Hotel 90goods->380 (220 gems)
Pizza Shop 100goods->430money (160 gems)
Pool Hall 120goods->440 (200 gems)
These are better (but also more expensive) than the Corporate Tower. Especially the Pizza Shop, with a ratio of 4.3.
A lot of Glass Towers is great, they give a solid 210 but can't be clicked very often.
Community buildings are better than they look. they only take 8 hours and can give energy back. The wind farm is the best (only one square) and the Opera and Aquarium are also great (they give 140 and 150.)
After testing I noticed that my wind farms give 2 energy back about 30% of the time. That means a wind farm is worth 2.5 times what it normal gives, or about 275 per click. That's even better than Glass Towers.