Cool to hear on the savings.
But now the team has to maintain two different deployment models, so you have account for the ongoing cost that your team now has to own and maintain two different processes of deployment (prod & staging).
The key element here is the need to continuously exercise both processes (Heroku + your staging server), to work out both processes & maintain familiarity on both.
Depending on the amount of staff involved in the above, it might eclipse the compute savings, but only OP knows those details. I'm sure they are a smart bunch.