To my knowledge there is no monetary cost to registering the QC client on a system. All that is being done is the registration of a DLL file. I can't speak to any potential security risks.
In order to register a DLL, the person executing the action must have administrative rights on the system where the DLL is being registered. This has nothing to do with the user's permissions within the QC application. It doesn't matter if the user is a Site Admin or a Project Admin. It only matters that they have admin rights directly on their workstation.
If your individual users can't be given administrator rights to the systems they work on, you might want to consider working with your IT support organization to deploy the QC Client Registration as an MSI package. This first reference to this in in the product Installation Guide in Chapter 10 under System Configurations > Additional Considerations. Make a note though that every time an ALM/QC patch is applied to the ALM/QC server, the QC client is updated. So the MSI would have to be rebuilt and redeployed.
In order to register a DLL, the person executing the action must have administrative rights on the system where the DLL is being registered. This has nothing to do with the user's permissions within the QC application. It doesn't matter if the user is a Site Admin or a Project Admin. It only matters that they have admin rights directly on their workstation.
If your individual users can't be given administrator rights to the systems they work on, you might want to consider working with your IT support organization to deploy the QC Client Registration as an MSI package. This first reference to this in in the product Installation Guide in Chapter 10 under System Configurations > Additional Considerations. Make a note though that every time an ALM/QC patch is applied to the ALM/QC server, the QC client is updated. So the MSI would have to be rebuilt and redeployed.