According to definition on MSDN: "ISNUMERIC returns 1 when the input expression evaluates to a valid numeric data type. Valid numeric data types include the following:
int, numeric, bigint, money, smallint, smallmoney, tinyint, float, decimal, real"
Comma is allowed in Money, hence it is returning 1. It will also return 1 if you +, - £ etc signs in the input.
Hope that helps. If it does, mark it as answer/upvote.