var varFormaPago = document.querySelector("[name=MedioPago]:checked").value;
alert(varFormaPago);
<input type = "radio" name = "MedioPago" value = "100" /> 100 <input type = "radio" name = "MedioPago" value = "200" checked /> 200 <input type = "radio" name = "MedioPago" value = "300" /> 300