Thanks for the question, Stuart!

To expand a bit on the explanation, ISNUMERIC determines if a string value can be converted to a numeric data type - any numeric data type. But it does not specify which data type the expression can be converted to.

The dollar and pound sign are valid in conversion from string to money or smallmoney (normally followed by the actual amount); the dot is valid in conversion from string to any non-integer data type (normally preceded by the whole part and followed by the fractional part). Without the numbers, the conversion will result, as Stuart already wrote, in the value zero.