First you should add labels for your inputs, they are linked to the input with ids
<label for="priceInput">Price:</label><input id="priceInput" type=text name=price><br>
<label for="taxInput">Tax:</label><input id="taxInput" type=text name=tax>
And in your jquery:
//Function is executed when the priceInput is changed, so when you enter a value
$("#priceInput").change(function(){
//Test if the value of the input is < 2000 and >0
if (($( "#priceInput" ).val()<2000) && ($("#priceInput").val()>0)) {
//Set the value of the tax input to 20
$("#taxInput").val(20)
}
//test if tyhe value is >2000 and <5000
else if(($( "#priceInput" ).val()<5000) && ($("#priceInput").val()>2000)){
$("#taxInput").val(35)
}
else{
$("#taxInput").val("")
}
});
JSFiddle link