Upgrades — spending coins with the server checking the bill

Your clicker makes coins. Now it needs somewhere to spend them — an upgrades shop. Buying has one rule, and it's the security rule again: the server checks the bill. A player can say "I bought the multiplier," but only the server knows if they could afford it.

#Where the upgrade lives

The upgrade level is real data, so it lives on the player's leaderstats — just like coins:

local multiplier = Instance.new("NumberValue")
multiplier.Name = "Multiplier"
multiplier.Value = 1
multiplier.Parent = stats -- your leaderstats folder

Add this in the same PlayerAdded block that makes Coins. Now every player has a Multiplier stat. The leaderboard will even show it, which is fun — but our real use is in the click math.

#The purchase flow

A purchase is a three-step conversation over a RemoteEvent:

1. Client: "I'd like to buy the multiplier" → FireServer("multiplier").
2. Server: check the player has enough coins → if yes, subtract and raise the level → tell everyone the news.
3. Client: update the UI with the new coins and multiplier.

Notice the client sends an intent ("multiplier"), never a price. The server owns the pricing.

#Server: the checkout counter

local purchaseEvent = ReplicatedStorage.BuyUpgrade
local UPGRADE_BASE_COST = 25

purchaseEvent.OnServerEvent:Connect(function(player, upgradeId)
    if upgradeId ~= "multiplier" then
        return -- don't recognize it, ignore it
    end

    local coins = player.leaderstats.Coins
    local multiplier = player.leaderstats.Multiplier

    local nextLevel = multiplier.Value + 1
    local cost = UPGRADE_BASE_COST * nextLevel

    if coins.Value >= cost then
        coins.Value = coins.Value - cost
        multiplier.Value = nextLevel
        purchaseEvent:FireClient(player, coins.Value, multiplier.Value)
    else
        purchaseEvent:FireClient(player, coins.Value, multiplier.Value)
    end
end)

Two checks before spending:

• if upgradeId ~= "multiplier" — reject anything it doesn't understand.
• if coins.Value >= cost — the whole point. The client may claim a purchase; the server checks the wallet.

The cost grows with level (cost = base * nextLevel) — the classic formula that makes early upgrades cheap and late ones expensive.

-- guard the purchase

#Applying the multiplier to clicks

Update the click handler on the server:

clickEvent.OnServerEvent:Connect(function(player)
    local coins = player.leaderstats.Coins
    local multiplier = player.leaderstats.Multiplier

    if coins then
        coins.Value = coins.Value + (1 * multiplier.Value)
        clickEvent:FireClient(player, coins.Value)
    end
end)

Clicks now earn 1 × multiplier. Buy a few levels and the clicker visibly accelerates. You have an economy.

#Common mistakes

• Checking the price on the client. "If coins >= cost" in a LocalScript is a sign on a gate that doesn't exist. Price checks belong on the server.
• Trusting upgradeId blindly. A handler that treats any string as a valid purchase is a free-store. Reject unknowns.
• Letting the client pick the price. The client never passes cost. It passes an intent; the server prices it.
• Forgetting to update the client after a refusal. FireClient on both branches so the UI shows the true state.

Next: the shop panel that puts this purchase in the player's hands.