We're trying to co-author excel workbooks but only have Standard CAL/Sharepoint 2010 Standard Server license. Its my research so far that to have co-authoring for Excel, we would need to use Excel Web Apps which needs Excel Calculation Service (subset service of Excel Services). Thus, we're out of luck. Can anyone confirm my conclusion?
Are you meaning co-authoring as in Google (Google Documents)? As far as I know it is not possible for multiple people to work on the same excel document at the same time. If one user opens it in edit mode it will get locked.
The excel service app is available without the office web apps installed and you can already display and edit documents with it. Office Web Apps adds this for: MS Word and MS PowerPoint.
Have a look at the following page of microsoft regarding co-authoring in SharePoint 2010: http://technet.microsoft.com/en-us/library/ff718249%28v=office.14%29.aspx