By default players have a "Basic Wallet" which can only hold up to $5000.
Players may purchase larger wallets to hold more money, but someone cannot for example purchase a Champions Wallet when they only have a Basic Wallet.
When someone tries to obtain money with a full wallet, it will tell them in chat "Your wallet cannot fit any more money! Upgrade it with /wallets upgrade!"
"/Wallets help" will bring up:
"/wallets upgrade" Purchases the next wallet if you have enough money and tells you: "you just upgraded your <old wallet> to a <new wallet>!"
"/wallets info" Tells you what wallet you have, how much it holds, and what the next wallet is called
Wallets Obtainable To Normal Players:
Basic Wallet - obtained by reading rules, holds $5000
Trainer Wallet - costs $2500, holds $20 000
Ace Trainer Wallet - costs $15 000, holds $50 000
Champions Wallet - costs $35 000, holds $250 000
Special Wallets:
Gym Leaders Wallet - holds $60 000 (exclusively for gym leaders)
Donator Wallet - holds $500 000 (any donation of $10+ gets this wallet)
It's quite a simple idea, but I think it would be useful to prevent a single person from obtaining enough money to destroy the economy, and it would be a fun goal to get the biggest wallet possible! It also provides another feature for donators, the $500 000 wallet.
EDIT: Yes I do realize this will force people to have at least a trainers wallet before purchasing a house, but I don't see a problem in that. :P