Generally, there are several options for getting a second opinion on an SAP consulting firm fees:
1. Ask another SAP consulting company (a direct competitor of the consulting firm with which you signed a contract) for an estimate of the same project / task.
2. If you have a contract for software licenses directly with SAP SE, you can ask their expert opinion about the fees of your SAP consulting firm.
3. You can hire an experienced SAP consultant or an SAP architect as an employee in your company and ask them to validate estimates of your SAP consulting provider.
If you intend to work with that SAP consulting firm long term, the option 3 might be the most cost effective option.