What you have written will not work, for several reasons. Firstly, as the name suggests, getElementById gets an element based on the value of the id attribute. You haven't given your input elements an id attribute, so that's not going to work.
Secondly, document.getElementById('someId') returns an element, but you want the value. You can use the value property to get it:
var price1 = parseInt(document.getElementById("test1").value, 10);
var price2 = parseInt(document.getElementById("test2").value, 10);
alert(price1 + price2);
This will work with the following HTML:
<input type="text" name="test" id="test1" value="300" />
<input type="text" name="test" id="test2" value="500" />
Note the use of parseInt. The value property returns a String, so we use parseInt to attempt to parse that into a Number. If you don't use it, price1 + price2 will simply concatenate the strings.