Something like this developed into a function might do the trick.

Ex.

DECLARE @var varchar(40)

declare @tempVar varchar(40)



set @var = '48501239955.486'



set @tempVar = rtrim(ltrim(reverse(@var)))



select @var = reverse(left(@tempVar,charindex('.',@tempVar))), @tempVar = right(@tempVar,len(@tempVar) - charindex('.',@tempVar))



while len(@tempVar) > 3

begin

	select @var = ',' + reverse(left(@tempVar,3)) + @var,  @tempVar = right(@tempVar,len(@tempVar) - 3)

end



select @var = reverse(@tempVar) + @var



print @var