Combining numbers inside a gui text?

So I’m trying to add money when i attack, but it only adds the whole number every time.
Like this : 1010101010 Instead of : 10
next time : 20 : 30
etc…

here’s my lines:

public float gains = 10.0f;
public string money = "";

void Start () 
	{	
		money  = "$ ";
	}

void OnGUI ()
	{
		
		money = GUI.TextField(new Rect(10, 10, 200, 20), money, 25);
	}

void Attack ()
{
      money += gains;
}

Now… My Attack function does not look like that, but you get the point.

public float gains = 10.0f;
public float money = 0;

void Start () 
{   
}
 
void OnGUI ()
{
   GUI.TextField(new Rect(10, 10, 200, 20), "$" + money.ToString(), 25);
}
 
void Attack ()
{
      money += gains;
}