Also maybe a more important question, what's the appeal of speeding up your progress with real money?
The appeal of a freemium game is the feeling of exploration even if its exploration in a contained space. Think of it as the feeling of creating change, and you are the agent of the change. Kinda like the idea of imagination with legos and while imagination is cool, it is also cool to start making it real.
You first need to place a person into this kinda of world, this kidna of mindset prior to even them thinking it is a game that costs money.
Once they are already engrossed in the mental simulation. Once it has become a game and they feel a slight rush. You then remind them that they can speed up the simulation, create more profound change, by using these resources, these credits, just like these other resources the person is spending all the time that the game gives for free, or the game gives if you shake the tree, or you pick them up from the ground.
But you get an even more profound change by spending another type of resource, a resource that is a gem, or coin, or shiny gold bricks, but in reality this resource is you spending outside money just like you were putting dollar bills on the table and handing them over, or if you were swiping with your credit card.
How freemium games work is they are a mental simulation that is too engrossing, and they separate you from the feelings of your body, they also separate you from the feelings of the outside world, and you do not think of this is costing you money, money that was meant to do X in the outside world. Now you just think I need more gold coins to create more change in this virtual world.
Put another way think of it as someone "hacking your mind" and instead of you living in the real world, part of your mind thinks it is in a virtual world. Put another way this is what the art of suggestion, some people call it the art of hypnotic suggestion. Where you are not a sleep but part of your senses are dulled, but another part is hyper aware, and the suggestion is so obvious that you do not think the counter narrative up to solve the problem.
Freemium games and whales will not work in the majority of the population. Most of the population may spend a small amount, but it is to unlock features that are not possible in another way, but the whales spend money not to unlock features but instead to speed up the game, as a force multiplier as an analogy. This is similar to how certain people are more suspectible to certain styles of marketing, or certain people are more suspectible to hypnosis, or more people are more suspectible to the stroop "interference" effect
https://en.wikipedia.org/wiki/Stroop_effect.
For the people who are not aware of the stroop effect. Imagine that you have to write down the name of a color when you see the word of a color on a computer screen. If the screen flashes blue the word you write down blue. Now imagine what happens when they do the word blue, the word red, the word green both in the word but also match the color blue, red, and green to those words. Now in the middle of the "game" they started making the word blue the color red, and the word green the color blue to start confusing you. This is what the stroop effect is, sometimes called the stroop interference effect. It has to do with processing speed, automaticity of thought, selective attention where you ignore information, response control / inhibition, and it can involved with parallel information distributed processing when you add more rules. Like you are supposed to ring a bell whenever you see the color blue, but you are not supposed to ring the bell when you see the word blue and the color is also blue. Put another way the stroop effect is a fancy and more complicated way of doing simon says but with different senses and different rules on what is the triggering factor for simon.
South park did a wonderful job explaining this phenomenon with their episode freemium is not free (season 18 episode 6)
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I am not against the idea of freenium games, freenium games are a good form of free trial for things like games, but also stuff that are not games like software. That said I believe laws should be written where the app developer is forced to put in the app preferences in some place where the user can state before hand, I want to spend only $20 a month, or $200 a month and after that limit has been surpassed the user has to re-enter their credit card information instead of the game taking the information directly from the already entered information from google, apple, etc. In other words they user can change their mind, but after that preset limit prior to them starting the game of $20 a month, or $200 a month they are "automatically cut off" unless they choose to re-engage.
Pretty much you re-entering your credit card information is forcing you to exist the mental simulation and force you to re-make a choice once again whether you want to continue or not.