They way we can do it is simple

Take its current value, and add to it. Do this by doing the following:

x = 1

x = x + 1

print(x)

Output:

2

Let's put this into your code on line 13

Character:WaitForChild("Humanoid").WalkSpeed = Character.Humanoid.Walkspeed + WalkSpeedAmount

Before I go on, I urge you to check out some scripting tutorials. Things such as this should be learned before you attempt to do advanced stuff. Also, I'd recommend comprehending your scripts; I assume you copied this from somewhere, as what you're asking for is done on line 21 already.

But I digress. You would add the health/speed to the boost.

local WalkSpeedID = 361576482
local HealthBoostID = 361576482
local Cash1ID = 361576482

local WalkSpeedAmount = 10
local HealthBoostAmount = 20
local Cash1Amount = 5000

local MarketPlace = game:GetService("MarketplaceService")
game.Players.PlayerAdded:connect(function(Player)
    Player.CharacterAdded:connect(function(Character)
        if MarketPlace:PlayerOwnsAsset(Player, WalkSpeedID) then
            Character:WaitForChild("Humanoid").WalkSpeed = Character:WaitForChild("Humanoid").WalkSpeed + WalkSpeedAmount
        end
        if MarketPlace:PlayerOwnsAsset(Player, HealthBoostID) then
            Character:WaitForChild("Humanoid").MaxHealth = Character:WaitForChild("Humanoid").MaxHealth + HealthBoostAmount
        end
    end)
    local Money = game.ServerStorage.PlayerMoney:WaitForChild(Player.Name)
    if MarketPlace:PlayerOwnsAsset(Player, Cash1ID) then
        Money.Value = Money.Value + Cash1Amount
    end
end)