How to add or subtract number to html element with jquery

I am trying to add/subtract from a number inside a span tag. This is depenent on a checkbox. Here is the jquery I'm trying:

//If checked
var $newprice = $("#totalprice").val() + 299;               
$("#totalprice").text($newprice);

//If NOT checked
var $newprice = $("#totalprice").val() - 299;               
$("#totalprice").text($newprice);

If I check the box, it adds 299, but unchecked changes the number to -299.