why would this remote event script work? (a nil value)

Never trust the client! This script is completely flawed, as you said in the previous answer's comment "people can exploit it easily which is why i want to add in it .Value". This won't make any difference, anyone with RemoteSpy and basic lua knowledge can pass their own values through easily. I suggest you simply fire the server in the client with no parameters, then have their money and the price checked server side. I'm not sure why anyone would ever have their price client sided anyways because a exploiter can easily reduce it or make it negative and give their selves money. Anyways, I'm done rambling let me explain to you why its not working.

As the other person answered, you should add .Value to the end of the script, now before you comment the exact same thing you commented to him hear me out. I understand you are trying to make it so that its only passing the NumberValue, but in theory if I was an exploiter couldn't I just make my own NumberValues and pass it through?

I suggest you create a system where every player's money is stored into values in a folder in ServerStorage and then when it needs to be changed, the number in ServerStorage is changed and then a .Changed function fires the client to update whatever client sided things, like UI, need to be changed.

Remember: The exploiter will always have the upper hand, so make sure to do as much as you can to stop them.

So coming from what you heard, you are going to have to send the server the value from the client anyway, as the server can't open it up. What I'd do is send the server the value, but to use some sort of sanity check. (I'll explain later)

Local:

script.Parent.MouseButton1Click:Connect(function() --Use :Connect instead.
    game.ReplicatedStorage.Event:FireServer(script.Parent.Money.Value, script.Parent.Price.Value) 

Server:

--Create another RemoteEvent, with a different name
local checked = false
game.ReplicatedStorage.Event.OnServerEvent:Connect(function(player, money, price)
checked = true
    game.ReplicatedStorage.Checking:FireClient(player, money, price) --This sends another message to the client.
   --Where's the other part? We'll get to it later.
    wait(2)
checked = false
end)

Other Local (Should be in the same local script as the other.

game.ReplicatedStorage.Checking.OnClientEvent:Connect(function(player, money, price)

    if money == script.Parent.Money.Value and price == script.Parent.Price.Value then
        game.ReplicatedStorage.Checking:FireServer(player, money, price)

end
end)

Other Server (Should be same script as the other server script part)

game.ReplicatedStorage.Checking.OnServerEvent:Connect(function(player, money, price)
if checked == true then
 if money >= price then
        print("Bought")
    else
        print("Not Bought")
    end
end
checked = false
end)

So what did we do? To some, this is called a sanity check. These are used to try to make an exploiter's achievement harder. Note that exploiters will always have the upper hand, but can be dealt with. (They can read and alter client code, but with the right set-up, we can get back at 'em.)

In our first part of the local script, we just sent the values. However, they'll be check later on. In the first part of the server script, it actually sends the information right back to the client with a different RemoteEvent, and this is where we're able to compare information. We also set up a variable that's set to true when the RemoteEvent is passed to the server (Remember, exploiters can't edit server code!) On the last part of the local script, it compares the numbers the server has and the numbers you currently have. If they match, huzzah, they'll go up to the server again. If it doesn't, it'll reject it and stop the pro if money >= price then print("Bought") else print("Not Bought") end process. And then on the last part of the server, it'll see if that checked value is right, making sure a hacker didn't send an event up from the client directly. After that, it'll then mark your purchase. Hopefully this isn't too confusing, and if you have any questions, I'll be glad to help.

I think the problem could be when you're sending a value through the RemoteEvent, instead of:

game.ReplicatedStorage.Event:FireServer(script.Parent.Money, script.Parent.Price)

try to put

game.ReplicatedStorage.Event:FireServer(script.Parent.Money.Value, script.Parent.Price.Value)

If it doesn't work, then I don't know how to solve it.

Hope it works!