I think you should use this approch:
var list = new List<Tuple<TextBox, decimal>>(){
Tuple.Create(textBox1, 0.05m),
Tuple.Create(textBox2, 0.1m),
Tuple.Create(textBox3, 0.2m),
Tuple.Create(textBox4, 0.5m),
Tuple.Create(textBox5, 1m),
Tuple.Create(textBox6, 2m),
Tuple.Create(textBox7, 5m),
Tuple.Create(textBox8, 10m),
Tuple.Create(textBox9, 20m),
Tuple.Create(textBox10, 50m),
Tuple.Create(textBox11, 100m),
};
decimal sum = list.Sum(tuple =>{
int a = 0;
int.TryParse(tuple.Item1.Text, out a);
return a * tuple.Item2;
});