How to do a Sum using Dynamic LINQ

I have the following which works brilliantly with the dynamic linq library:

string where = "Price < 5";
string orderby = "BookID ASC";
IQueryable<T> MyDataQueryable = _DataRawBase.AsQueryable<T>();
MyDataQueryable = MyDataQueryable.Where(where).OrderBy(orderby);

Now I want to query the MyDataQueryable to do a SUM (and perhaps Average) of certain fields.

How would I go about this?

Something like:

double mysum = MyDataQueryable.Sum("Price");  

would be nice...