By Michael S. Derby NEW YORK, Jan 22 (Reuters) - There's scant evidence so far that U.S. President Donald Trump's bid to make housing more affordable by purchasing mortgage-backed bonds has had its desired effect, and to the extent it has, broader geopolitical anxieties driven by his administration look set to make borrowing more expensive over time. The targeted $200 billion of ​purchases appears likely to make borrowing costs cheaper only at the margin, while a wide range of experts agree the key to m ...