There sure is.
= "Page: " + PageNumber("group1", Fields.salesorderID) + " of " + PageCount("group1", Fields.SalesorderID)
That is the expression that I use in my reports.
So typically you would drop this in the page footer as a text box.
Group1 = what ever the name of your group is that you are grouping the invoice on.
SalesOrderID = this is my transactionID from my database. You will need to change this to suit your database.
Let me know how you make out as it took me a while to figure this one out.