I obviously can’t speak to your specific bill. When I used to have visibility into such things the vast majority of people who said “my Heroku bill is huge!” we’re actually giving very little of it to Heroku. Most of it was add-ons for metrics, logging, error tracking, etc.

If you’ve not already gone through the line items I’d recommend it. You might find that replatforming isn’t as cheap as you’d expect, unless that also included ditching the multitude of other SaaS products the team has accumulated.