IPS Operators are typically run as not-for-profit utilities. They usually earn revenue from a mix of annual fees and per-transaction charges.
The Nexus Scheme would not dictate how each IPSO should charge its own participants (PSPs and SAPs) for the service the IPSO provides to them.
Since IPS Operators usually only process domestic payments, connecting to Nexus could allow them to increase the volume of payments that they process. Cross-border payments are more complex to process than domestic payments, so it would be expected for an IPS Operator to charge a higher per-transaction fee for a cross-border payment, and/or an additional annual fee for enabling the cross-border service for a PSP.
On the costs side, an IPSO would need to pay a membership fee and/or per-transaction fee to the entity that operates the Nexus network. They would also have to make some initial changes to their own system as they integrate to Nexus.