How to make a buy zone area that costs 100?

Local scripts change things on the client (players computer only visible to them) Scripts change things on the server (which everyone can see)

I recommend having the "script" which checks of the player can afford the plot to be placed inside the Zone object.

local function getPlayerFromCharacter(character)
	for _, player in pairs(game:GetService("Players"):GetPlayers()) do
		if player.Character == character then
			return player
		end
	end
end

Zone.Touched:Connect(function(hit)
    If hit.parent.Humanoid then
        local player = getPlayerFromCharacter(hit.parent)
        if (the players leaderboard allows it) then
            --do your stuff
        end
    end
end)