You're not being silly.  It's a very common constraint of Crystal that some formulas simply can't be aggregated.  This is especially true where running totals get involved.  (The reason for this has to do with the order in which Crystal calculates such things.)

But, you can certainly create a custom formula to do the work for you.  Try something like:

If ext = "yes" then
formula = Sum ({NewRate}*{Hours})
Else
formula = Sum({OldRate}*{Hours})

Hmm.  I think I led you somewhat astray.

To do a running total, create a global variable.  Actually, in this case, create two global variables.  We'll call them NewRateTotal and OldRateTotal.

Create a formula, that should look something like:

WhilePrintingRecords;

Global NumberVar NewRateTotal;
Global NumberVar OldRateTotal;

If {ext} = "yes" Then
NewRateTotal = NewRateTotal + {NewRate}*{Hours}
Else
OldRateTotal = OldRateTotal + {OldRate}*{Hours};

Then, in your group footer, you can put something like:

WhilePrintingRecords;



Global NumberVar NewRateTotal;

Global NumberVar NewRateForReport;


NewRateTotal;
NewRateForReport = NewRateForReport + NewRateTotal;
NewRateTotal = 0;

And, another one for OldRateTotal.  Then, in your report footer, put:

WhilePrintingRecords;

Global NumberVar NewRateForReport;

NewRateForReport;

However, given your specific requirements, there may be an even easier way.  Try in your report footer:

SUM (IIF({ext} = "yes",{NewRate}*{Hours},{OldRate}*{Hours}))

That will sum everything up in one grand total.  (If you want to just the NewRate sum, just replace the OldRate bit with 0, and vice versa.)