Hi,
Customer expected price means user will be entering some value in selection screen and based on it you need to do some calculation with the SAP calculated price.
If that is what your requirement is then I assume SAP calculated price you must be getting from the system and itself.
For user input price--Create a formula variable Processing by manual input--Variable should be ready for input and Currency unit tab--Dimension as Number.
In case your requirement is something different then what i deciphered then please share it with some appropriate example.
Regards,
AL