Islands of Adventure is also very very high-tech... that's going to the extreme.
I would estimate about a few hundred million to build all the land, the theming, attractions, landscaping, and rides.
This would be for about 100 acres on average, starting with a decent ride & coaster count.
If you are just starting off... a park can be pretty cheap. Find a cheap but spacy location that's easily accessable, and get the land for about $3000 per acre x 100. Buy about 10 + used rides for around $100,000 to $500,000, and a couple feature rides for a few million each, and you can start a half-way decent park for under 50 million which isn't bad since some lotto's have jackpots for more than that amount!!! :)
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The Great Escape... Soon to become Six Flags Adrondiak Escape!!!