Solstice has no integration with economy APIs whatsoever and cannot support command execution costs.
Describe the solution you'd like
I found this API (https://github.com/Patbox/common-economy-api) but it seems to be unmaintained (last commit was 3 years).
Other projects that I found:
If these projects end up not working, I may consider developing the API for it, but that should be avoided if possible.
As of the costs, see #47, the pre-run event would come in handy here.