Oh, yes. I know you did not mean it but I felt like a liar now

But actually this just proves my point. They only sell coins and double coin (Please don't find anything else

)
Look at the characters. There is a character (Prince or something) worth about one million coins. Tons of stuff actually and only a little of them requires small amounts of coins. Most of them are expensive.
When you look at them, you see that they are expensive. But coins are not hard to earn through game play. This is the important part in my opinion. See, if you are patient, you enjoy the game and earn coins. But if not, then just go buy some coins.
Let me explain you the game a little bit more, so you can see why I think it is one of the best in-app purchase implementations in my opinion.
1- Coins are earned in so many ways. If you play a couple of times, you will see that you can collect boxes too, which reveal from game items to coins. And I just got 100000 coins from a box. So they seem like they only want you to buy coins if you are
really impatient.
2- They have mission sets and after you complete, you get those boxes too.
3- They have some characters and surfboards which you can only unlock by playing the game (Collecting game items such as radios, UFOs, guitars). This implicitly tells a player that they don't need to buy coins to enjoy the 'special' stuff.
4- This is a downside for them but I'll say: The absurdly high priced characters do not have any exclusive features other than the image. For instance, I could play the game a lot more if I knew (Maybe it has, but they don't mention it) that the one million coin character had some kind of special attribute, like converting an upgrade item to any upgrade item you want (Convert star to coin manget, so I can get more coin). I would play until I get to that character and see what it does.
But they have 2-3 skateboards which have different attributes.
5- There is always one or more short-term, middle-term and long-term goal to reach. Missions provide short and sometimes middle term goals. You complete them, you get stuff. Expensive characters and special characters provide the long-term goals. So, you never get bored.
For example, Temple Run had some objectives like getting 10000000 points in one run etc. If you can't get to that objective, you get bored. And at least for me, it was a hard one. And there were no middle term objectives. Hence, I don't have Temple Run on my phone now.
Actually, Subway Surfers has 20-30 missions which are randomly picked, but that keeps the game fresh.
6- Besides bringing always-fresh air to the game, actually those short-term objectives keep a player in the game. And your best bet to sell something (even your cheapest thing) to a player is to keep the player in the game.
If he plays it for 10 minutes per day, he will think about it for another 10 minutes later in the day. If he plays 2 hours, then thinking time is 2 hours. I think I don't need to explain that if you think enough, anything can become an obsession

And they have a regular update schedule now. They "tour the world" if you read the description. They were in Rio and just when my brother got bored of Rio, they went to Rome
I might try to give you some ideas as a friend of a game freak (I am just game 'enthusiast'), if you could provide more information about the game.