How can I develop and support mainframe software without going broke?
One possibility would be to find a 3rd party business owning such infra and selling on-demand usage/access time/connectivity to it as a service for more convenient expenses. For example Mathru Technologies (found through plain google search).

Another possible approach is to obtain such services from strategic partners or maybe even your very customers. Maybe even for free, depending on the quality of your relationship.

You could also ask the mainframe community for advice and/or leads in this direction.