About the Mechanic
Purchasing Upgrades encourages players to practice a basic life skill - knowing how many resources you have on-hand, so that you can determine how much you can spend, how much you need to keep back, and further teaches the player to make plans to buy something expensive that the player will want.
You can't discuss purchases, without discussing currency; they're two sides of the same coin, and that pun was very intentional. Originally, the plan was for currency collection to be its own mechanic, but that's unrealistic, because the exact manner of collection is unimportant; the fact that you can obtain currency is what enables you to make a purchase in the first place.
This mechanic can be considered another sub-mechanic of Incrementing a Score - unlike Gaining Levels, where you don't actually lose any score, just have previously-used score hidden, you actually consume a score to buy in-game perks.
Purchases are a well-rounded mechanic because they define a number of things. Purchases define a success condition - if the player can buy what they want, they have succeeded. Purchases define a failure condition - if the player can't gain money in a reasonable amount of time, they need to re-assess their strategy, so that they are gratified in a time that they feel is reasonable. Purchases define an implicit goal for the player; that expensive object would be really helpful! Finally, purchases provide for feedback of both progress and success or failure, in the form of handy numbers.
This gets to why the currency is important - the purchase itself is the end result of the mechanic. In order to afford the purchase that a player wants, the player has to find a means to accrue a suitable currency.
A player who feels secure with Gathering Currency and Purchasing Upgrades feels like they've mastered the economy of the work in question. Likely, the player has a gut feeling on what an adequate rate of gain 'should' feel like. The game designer best-serves their audience by erring slightly towards the longer-end of that gut feeling, so that the player can feel like their goal is met in a satisfying, but challenging, way.
Referee Information / Data Structure
Current Currency Amount
Event Currency Amount
Currency Gain Rate = Event Currency Amount / Unit Time
Time to Purchase = Item Cost / Currency Gain Rate
Victory: Current Currency Amount >= Item Cost
Failure: Current Currency Amount < Item Cost
When Is This Mechanic Engaging?
- When the player character is interacting with NPCs or other players; this simulates real-life economies, which is a practical point of the mechanic
- When you need a way to give the player a more implicit goal - buying something you want isn't necessarily something that needs to be written in a quest log, and is something you can usually rely on a player to remember, because it suits their interests.
- When you need to add some play time to your game. Time is the fundamental currency that players deal with, and in real life, what you are paid is proportional to the amount of time you invest into something. As Currency/Purchase systems are a simulation of real-life economies, this is a fact that, if properly balanced, actually improves your player's suspension of disbelief, because it's real and interacted with daily. It's provable and about as realistic as you can hope to get.
When Is This Mechanic Distracting?
- The player seldom encounters NPCs who are interested in trade. This is distracting, because this feels 'tacked on'. Economies are very much social interactions, even in the Internet age. I call this the Lone-Wolf Buyer Problem.
- The vendors do not offer items that are useful to the player. If the player has no reason to use a shop, purchasing items is a superfluous feature; they are better-served to simply get advantages in other ways that the work allows for. I call this the Knock-Off Rolex Problem.
- Similarly, money is too hard to come by. If players cannot accomplish their implicit goal in a reasonable amount of time, they will deem it a waste of time, and move on. After all - Time is the fundamental currency that we deal with our players in! I call this the Bare Coffers Problem.
- On a related note, money is too easy to get. If players can easily obtain things they want, there is no challenge, which does not help the player to feel as though they've mastered the economy of the work in question. I call this the Welfare Hero Problem.
Note: The reason I have names for each of the problems, is because many games use currencies and purchases as a mechanic; few use them effectively, including some very well-established games that are considered masterpieces.
- The Legend of Zelda: A Link to the Past is a prime example of the Knock-Off Rolex Problem. The most useful and expensive purchase in the entire game occurs at the very beginning of the work, one of the four bottles used to store restorative items. Beyond that point, there's not really much of a point in collecting Rupees or making purchases.
- Final Fantasy XIII is a prime example of the Lone-Wolf Buyer Problem. Final Fantasy XIII is notorious for being barely recognizable as a JRPG in the first place. Through the game, the player will encounter random consoles that allow the protagonists to contact various Cocoon shops to purchase consumables, equipment, and crafting materials. It's fully possible to play through the game without using the shop feature of the terminals, at all. The only thing that the shop system in FFXIII got correct, was that if you know what to do and buy, you can create powerful advantages relatively early on by using the shop system.
- The game Half-Minute Hero handles money quite effectively in the Hero 30 mode. The main way the character progresses is by buying armors and weapons, sometimes that have special effects including insta-killing certain enemy types, map mobility options like swimming in water, and more. Notably, HMH tackles the Bare Coffers Problem in a specific level by setting the problem up - no one in the level has any money, including random encounters - and the player must complete quests while rewinding time in order to gain money and level up enough to defeat the stage's Evil Lord.
- Final Fantasy VII succumbs to the Welfare Hero Problem. The game goes out of its way to ensure that the party has ample access to money, which makes the purchases nearly useless for casual play; grinding Materia, and exploring for special, unbuyable items is more useful to the player to advance through the work. This applies with a caveat, however - in that work, shops don't fulfill their 'usual' function, which is to create a goal. Instead, shops serve player customizability. Virtually all Materia is available throughout Gaia through shops, which means if you had to ditch some Materia along the way (you have ample carrying space too), you can easily re-buy what you need, to accomplish a build on a character.