Warframe, one of the most popular MMO games that has been around for 7 years, was just ranked as one of the top sellers of 2019 on Steam. While the game itself is free-to-play, several items or services in the game can be purchased through premiere currency or directly with real money. I started playing Warframe about 40 days ago and have spent 123 hours in it (I know, it's a lot, thanks to the quarantine). While I've heard some people "accuse" the game for being pay-to-win, that is not what my experience told me. In fact, the purchasable items and services are designed in such an interesting way that sometimes incentivize players to pay and at the same time won't make them feel they have to keep tossing in money to be powerful. They achieved such balance by monetizing on a mixture of different player emotions.
Platinum Discount: Urgency, Delight of A Cheaper Price, and Freedom of Choice
In Warframe, "platinum" is a primary currency that players mostly need to pay real money for, starting at 4.99 USD for 75 platinum. You can spend platinum to obtain weapons or warframes instantly, as well as on other services. As part of the daily login bonus, players can occasionally get a discount (from 25% off to 75% off) on their next platinum purchase within 48 hours.
Similar to deals on any other items in video games, a timed discount makes players think they would gain more value through purchase simply because they are now paying less for the same thing. And the limited time period would sometimes make them regret if they miss the deal and therefore lead them to purchase. For this platinum discount in Warframe specifically, it also gives players a sense of freedom because they can spend the platinum on whatever they want, as opposed to in some other games where the discount is on specific in-game items. In this sense, the timed discount has a more neutral value since how much players can make out of this deal depends on themselves, as opposed to in some games where players have to carefully evaluate the items before purchasing.
Speaking from my personal experience with those platinum discount, they are more of a chance for me to stack up platinum, which I know would be valuable for a lot of situations, as opposed to instances where I might be "lured" to purchase things I later realize I don't need.
Weapon / Warframe Slots: Satisfaction of Seeing Progress, and Freedom to Have Options
In Warframe, although there's no limit on the inventory, players can only have a limited number of weapons and warframes in their arsenal. If they want to expand their weapon and warframe slots, they'd have to pay with platinum (technically players can also obtain free slots through some events or leveling up, but they are very rare).
Charging for more capacities of weapons is not rare in video game. Some games will even only allow you to carry certain powerful weapons after you pay. In Warframe, however, the slots are not tied into how powerful you can get. A player can totally play and experience all weapons by selling weapons they don't need anymore. Selling weapons at max levels would neither have any punishment. And the same goes for the warframes too.
If that's the case, why would players want to spend premiere currency on weapon and warframe slots then? One reason is the satisfaction of seeing tangible progress. A collection of weapons and warframes represents how much a player has progressed in the game. Being able to visually see that progress is satisfactory and would even drive players to achieve more. Another reason is the option to choose different weapons. Different weapons and different warframes can lead to very different play styles. If players have a wider range of weapons and warframes to choose from, they'll have more freedom in tailoring their own gameplay experience.
Paying for Colors: Personalized Experience
In Warframe, you can customize your weapons and warframes with colors and decorations. While the game gave you a very small amount of those for free at the beginning, players will have to pay for most color palettes and decorations with the premiere currency--platinum.
Weapons and warframes in the game are highly customizable as players can choose different colors for different parts on them. Players have very strong temptation to put on their own color combination because everyone wants to be unique, and they want to have a personalized experience with an avatar that represents themselves. This motivation will drive them to pay for the color options, which is purely for look and won't affect stats at all.
Among all monetization in Warframe, I found this one somewhat most frustrating. The price of one color palette (which contains about 90 colors) is much higher than a lot of other resources that would actually affect gameplay. And you would only have access to 18 colors (which don't even cover the basic color schemes) if you choose not to purchase at all.