Good Question:
Also
DECLARE @PrincipalAmount VARCHAR(15)
SET @PrincipalAmount = '£700,000'
SELECT ISNUMERIC(@PrincipalAmount)
returns 1 as does
DECLARE @PrincipalAmount VARCHAR(15)
SET @PrincipalAmount = '$700,000'
SELECT ISNUMERIC(@PrincipalAmount)
See http://msdn.microsoft.com/en-us/library/ms188688.aspx for other recognised currency symbols