@robert_wlliams you can start by debugging, so check your browser Network tab on what is the response from your request.. Is it a correct one?
You can also print to the console, in the success callback: console.log(data); and check what does that returns..
And third:
$data = Stock::where('price')->where('id',$request->id)->first();
is a wrong query, you have where('price') checking for no value at all, so remove that:
$data = Stock::where('id',$request->id)->first();