Thanks for your reply.
I actually found the solution on the Tip calculator youtube vid and tweaked it as follows
var Box_A = Number(Apperyio("Box_A").val());
var Box_B = Number(Apperyio("Box_B").val());
var Box_C = Box_A + Box_B;
//Apperyio('Box_C').text('£' + Box_C.toFixed(2));
this works but the problem I now have is as I mentioned before I receive prices into Box A from a 3rd API. The results are received and displayed as "£3.50" for example into Box A. When adding Box A and Box B, Box C displays "NaN". If I then manually delete the "£" out of Box A it works fine. So the "£" is causing the "NAN" problem. How can I configure it to ignore the "£" and just add the "3.50"?