Using the accepted answer from .NET String.Format() to add commas in thousands place for a number, use an if statement to control to returned format.
string str_Money = "";
if (money % != 0) // test for decimals
{
str_Money = string.Format("{0:n0}", money); // no decimals.
}
else
{
str_Money = string.Format("{0:n}", money);
}