I think your request would attract more attention by setting the status to 'unanswered' instead of 'assumed answered'.
Coming to your question:
You can use inter record function like PEEK() to do what you want, you just need to take care of the input records coming in correctly sorted:
(html, codepage is 1252, embedded labels, table is @1);
peek(cost)- cost as difference
ORDER BY time desc;
drop table INPUT;