The fee for a tax return is the same regardless of filing status --- ie the fee for a tax return filing jointly is the same as filing separately. However, if the couple decides to file separately (and both have U.S tax requirements), they will require two sets of tax returns.
The price for a married couple filing jointly is the same as for an individual Streamlined Procedure. However, if they have separately-owned foreign accounts and need duplicate sets of FBARs, additional FBAR forms may need to be prepared.