You can easily manage your prices for each one of your clients under People > Clients > Services.
Here you can define all the services you provide to this client and their prices. You can also add matches if you're working with a CAT tool.
These prices will be used to automatically calculate an income estimate for each job you enter in the system.
Please note that when an invoice has already been created for a job, it won't be automatically updated with the changes you might make afterwards at the client level concerning the prices of your services. In this case, you should delete the invoice and recreate it after you made the changes.