Use new "cost" field in usage APIs

The previous, manual calculation could be incorrect depending on
which SMS rates the free allowance was attributed to.

The new field also supersedes the old "letter_total" bolt-on so we
can get cost information consistently for both types.

tests/app/main/views/test_dashboard.py

@@ -1020,7 +1020,7 @@ def test_usage_page(
     assert '251,800 sent' in sms_column
     assert '250,000 free allowance' in sms_column
     assert '0 free allowance remaining' in sms_column
-    assert '£36.14 spent' in sms_column
+    assert '£29.85 spent' in sms_column
     assert '1,800 at 1.65 pence' in sms_column
 
     letter_column = normalize_spaces(annual_usage[2].text + annual_usage[5].text)
@@ -1855,7 +1855,7 @@ def test_service_dashboard_shows_usage(
     ) == (
         'Unlimited '
         'free email allowance '
-        '£36.14 '
+        '£29.85 '
         'spent on text messages '
         '£30.00 '
         'spent on letters'