Krarak - minimum with 18 chest needed is 7200 so higher than that as a base since you don't get one of those in every roll
I saw one calculation based on the chest average of 23 currency which was 9200. Keep in mind it is 20 flags 20 times.
That also depends on when the amount needed per grand prize goes up from 20 to 30 coral after a certain point, like with the past two events with evolving buildings. I cannot remember when the increase happened, though, but it was sometime between the 20th and 30th grand prize.
But anyway, it take 17 grand prizes to max out one Mermaid's Paradise, and then the 18th grand prize is a second Paradise.
Also, for anyone else going this far in the prize cycle, the in-between buildings do not follow the same order the second time through. The Trading Outpost was the 12th prize the first time through, but it shows up again right after the 10th artifact, as the 20th prize, instead of not until the 24th spot.
Also, back on the numbers, I got the 9th artifact after the 6th quest at the 64-coral level, so I had earned 7524 coral from quests, plus what I had picked up off the ground in almost 6 days, plus a handful of chest rewards that were coral, and zero from the crafting chest so far this time. So maybe 8000 coral from everything got me a maxed out building. And whether is stays at 64 coral per quest til the end, or loops back to the beginning again like others have said it does, I still have plenty of days left to max another Paradise, depending on when the increase from 20 pearls 30 kicks in, and whether I am feeling that ambitious. lol