One variable plus another variable

One variable plus another variable

Hi,
I'm using a jquery invoice script from css-tricks where I wanted to add Taxes to the total.
The problem I have with it that when I wanted to add one variable to another withe the + sign the variable are placed after each other...
You can see the script on this page .
The line which causes it is:
var inclbtwraw = $("#subtotal").html().replace("€ ","") + $("#btw").html().replace("€ ","");

thanks in advance for your time!
Chris