Have you tried NUM($(v_Terminal))*NUM(Hours)?
Probably $(v_Terminal) is causing the issue. Experiment with the following options:
$(=v_Terminal) * Hours
v_Terminal * Hours
Or when the v_Terminal is multiplied with multiple Hour fields, then use a function like SUM.
SUM($(v_Terminal) * Hours)
$(vTerminal) * SUM(Hours)
Check the above suggestion's i.e. The variable is returning the value you expect and also the are both numbers.
The next thign you have to take in to consideration is the Aggrations of the Hour field.
You will probably want something like
SUM(Hours)*$(v_Terminal) or SUM(Hours)*vTerminal - Depends on if your variable needs to be evaluated (dollar expanded)