Here's a cheat I use on my Android version of the game. I don't know if it works on the iPhone version, but it's worth a try.

Every time your island resort is upgraded, you get five shells, right? I use this to my advantage, using the Luxury points as the last point I get before I get upgraded. For example, if I need 25 points in each level, I'll buy whatever it takes to get my 25 points in Entertainment, Restaurant, and Lodging. But I'll only buy enough items to get 24 points in Luxury. When I buy an umbrella (adds one point, costs 200 coins), my level goes to 25, my resort is upgraded to Cozy, and I get five shells. If I sell that umbrella, my Luxury points go back down to 24 and I get 100 coins back. But I get to keep the shells!

Now, if I buy another umbrella right away, I can bring my points back up to 25, but I won't get another five shells. However, if I exit the game after selling the first umbrella, and then re-enter the game, my resort class is reset back to the lower level. Now, if I buy another umbrella, I get another five coins. I buy an umbrella, sell it, exit the game, and re-enter. I do that as many times as I can before my fingers get tired or my battery goes dead.

Naturally, you don't want your resort class to stay at Cozy all the time. Plus, when you upgrade your other items, you'll get Luxury points that will put you over the 25... so eventually you have to move up in resort classes anyway. But you can do this at each level. At each level, 75, 150, 250, 350, whatever, I keep my Luxury points at one point under what it takes to get upgraded. Then all I have to do is buy an umbrella and I get 5 shells. I sell it and go back down under the upgrade point, but I get to keep the shells.

50 shells end up costing 1000 coins. I buy an umbrella for 200 coins, but I get half of that back each time I sell it. After I've done that 10 times, I have 50 shells. I can then sell those 50 shells for 198,000 coins. Or I can use them to buy other stuff, or for instant upgrades.