Good question, and I wonder what are the costs associated with running your own mint, if any?

I believe the costs are fairly straightforward. Running the server where the mint is hosted and running the lightning node infrastructure to route payments in or out of your cashu mint.

Mints can take transaction fees (which is also necessary to prevent abuse). A mint can be part of a larger architecture. Generally speaking, the cost for running the mint itself is minimal compared to for example a Lightning node.