Why does the value change but the cost doesn't?

If I just leave the value at 17 or in the script put cost.Value = Max.Value *2 or something like that it will work. But for some reason it will not work like this. The original cost is 17 and the value changes but the cost stays the same. Help please

Button script and cost script

local player = game.Players.LocalPlayer


script.Parent.MouseButton1Click:Connect(function()
    local upgrade = game.ReplicatedStorage.Functions.UpgradeBackpack:InvokeServer()
    if upgrade == false then
        script.Parent.Text = "You need more coins!"
        wait(1)
        script.Parent.Text = "Upgrade"
    elseif upgrade == true then
        script.Parent.Text = "Upgraded!"
        wait(1)
        script.Parent.Parent.Parent.Parent.Parent.Parent.Visible = false
        script.Parent.Text = "Upgrade"
        local cost = player.UpgradeCosts.BackpackUpgrade
       if player.backpack.WeightLevel1.Value == 2 then
                cost.Value = 20
                player.backpack.Max.Value = 80
            player.backpack.NextUpgrade1.Value = 160
end
    if player.backpack.WeightLevel1.Value == 3 then
                cost.Value = 21
                player.backpack.Max.Value = 160
            player.backpack.NextUpgrade1.Value = 200
end
    if player.backpack.WeightLevel1.Value == 4 then
                cost.Value = 22
                player.backpack.Max.Value = 200
            player.backpack.NextUpgrade1.Value = 210
end
    if player.backpack.WeightLevel1.Value == 5 then
                cost.Value = 23
                player.backpack.Max.Value = 210
            player.backpack.NextUpgrade1.Value = 230
end
    if player.backpack.WeightLevel1.Value == 6 then
                cost.Value = 24
                player.backpack.Max.Value = 230
            player.backpack.NextUpgrade1.Value = 250
end
    if player.backpack.WeightLevel1.Value == 7 then
                cost.Value = 25
                player.backpack.Max.Value = 250
            player.backpack.NextUpgrade1.Value = 300
end
    if player.backpack.WeightLevel1.Value ==8 then
                cost.Value = 26
                player.backpack.Max.Value = 300
            player.backpack.NextUpgrade1.Value = 301
end
    if player.backpack.WeightLevel1.Value == 9 then
                cost.Value = 27
                player.backpack.Max.Value = 301
            player.backpack.NextUpgrade1.Value = 302
end
    if player.backpack.WeightLevel1.Value == 10 then
                cost.Value = 28
                player.backpack.Max.Value = 302
            player.backpack.NextUpgrade1.Value = 303
end
    if player.backpack.WeightLevel1.Value == 11 then
                cost.Value = 29
                player.backpack.Max.Value = 303
            player.backpack.NextUpgrade1.Value = 304
end
    if player.backpack.WeightLevel1.Value == 12 then
                cost.Value = 30
                player.backpack.Max.Value = 304
            player.backpack.NextUpgrade1.Value = 305
end
    if player.backpack.WeightLevel1.Value == 13 then
                cost.Value = 31
                player.backpack.Max.Value = 305
            player.backpack.NextUpgrade1.Value = 306
end
    if player.backpack.WeightLevel1.Value == 14 then
                cost.Value = 32
                player.backpack.Max.Value = 306
            player.backpack.NextUpgrade1.Value = 307
end
    if player.backpack.WeightLevel1.Value == 15 then
                cost.Value = 33
                player.backpack.Max.Value = 307
            player.backpack.NextUpgrade1.Value = 308
end
    if player.backpack.WeightLevel1.Value == 16 then
                cost.Value = 34
                player.backpack.Max.Value = 308
            player.backpack.NextUpgrade1.Value = 309
end
    if player.backpack.WeightLevel1.Value == 17 then
                cost.Value = 35
                player.backpack.Max.Value = 309
            player.backpack.NextUpgrade1.Value = 310
end
    if player.backpack.WeightLevel1.Value == 18 then
                cost.Value = 36
                player.backpack.Max.Value = 310
            player.backpack.NextUpgrade1.Value = 311
end
    if player.backpack.WeightLevel1.Value == 19 then
                cost.Value = 37
                player.backpack.Max.Value = 311
            player.backpack.NextUpgrade1.Value = 312
end
        if player.backpack.WeightLevel1.Value == 20 then 
            script.Parent.Text = "MAX!"
    script:Destroy()
    end
end
end)

upgrade script

UpgradeBackpack.OnServerInvoke = function(player)
    local cost = player.UpgradeCosts.BackpackUpgrade

    if cost.Value <= player.leaderstats["Coins"].Value then
            player.leaderstats["Coins"].Value = player.leaderstats["Coins"].Value -cost.Value 
            player.backpack.WeightLevel1.Value = player.backpack.WeightLevel1.Value + 1
            return true
    else
        return false
    end
end

Thanks