From Longman Dictionary of Contemporary Englishmount up phrasal verbINCREASE IN NUMBER OR AMOUNTto gradually increase in amount At £6 a ticket, the cost quickly mounts up. → mount→ See Verb table
Examples from the Corpus
mount up• She finally had to go back to work to pay the bills that were mounting up.• The national debt has mounted up, growing to $1 trillion since Clinton took office. 