Distributed App Data Sync (Interesting Design Problem)

I have a very interesting problem to solve and I really need your help before I take off down the wrong path.  I have been tasked with designing a software solution for a services company that sends crews to remote sites to do custom services.  Each crew member has a tablet pc that runs Win 7 (and soon 8).  There is a master server on the web that has WCF services to do CRUD stuff with a master db which is Sql 2008.  Now, these teams can be out for days and thus there will be a reliable local Win 7 pro machine that hosts a local db (Sql Express) and local WCF services for the field application (which is really a subset of the Master DB.  Not all the tables from the master internet web server are used locally).  
My problem is that I am unsure what the best way is to syncronize the data from the remote site to the master db on a daily or weekly (or whenever a Internet connection is available) basis.  The local 'server' will be a laptop so the field manager can take it to a hotel (or use ATT wireless) to access the internet when required.
I have attached a diagram that draws everything out pretty well.  I am under a real tight budget as this is a Non profit service.  Please suggest solutions and provide links if possible.  Thank you in advance.
DesignDiagram.gif