There is a lot of variability to consider but I think the largest costs would be the Robot licensing along with any Infrastructure and non-uipath OS/software licensing and not a single Orchestrator environment
UiPath makes recommendations for their hardware requirements at scale as well as suggestions on how to handle performance dips due to volume of robots such as reducing the typing of logging or adjusting the buffer size, but that is more on the database and connection pool then the Orchestrator Web Server.
As @bcorrea mention you can setup Orchestrator in a multi-node high-availability setup.
To my knowledge there is no additional charge on the UiPath side (in an Enterprise setup) as it is considered one production environment so the additional fees would be on the infrastructure hardware (machine hosts, load-balancers, etc. and licensing on the OS (Microsoft) along with. In the SaaS model form UiPath you are not paying for Orchestrator but rather a slight higher costs for the robot licenses.
There are also requirements for the Robots; however this largely depends on the processes that you are going to be automating and the applications in that process (Something that is going to only be automating a basic web application is vastly different from something that might be CPU or memory intensive), whether you are showing standard desktops/VMs running Windows 10 or host with Windows Server and if that Server is going to be configured with RDS/Terminal Services which has additional licensing costs from Microsoft.
Your best bet her is to size your hosts appropriately for the tasks that they will be handling and whether it will be a 1:1 machine/robot or 1:many machine/robot ratio.
I would suggest reaching out to UiPath directly and speak with one of their Pre-Technical sales reps to better understand the technical requirements and how that affects the pricing.