Same question. I even finished the Giant's mission, just in case that was what unlocked it, but nope. No such luck.

So its just like a regular obelisk? That's kind of disappointing.

Kinda sucks, cause now I have nothing to spend gold on. I'M BUYING A SOLAR BARGE!

Keep doing both. No spoilers, but it eventually comes to a logical conclusion.

I decorated with the cheapest clay pots up to 500, then just sold them all. Took time, but wasnt to terrible.

Yeah. There aren't nearly as many gold mines as you can build houses. I have about 40 houses of varying types, and only about 7-8 gold mines. For the short term, totally worth it to get the sun temple. Sphinx might become more valuable over time, but you can save up for that second!