= Solution
A stationary evanescent wave does not assign a classical trajectory or a real local speed inside the forbidden region. The <dwell time in a semi-infinite potential step> measures probability stored in $x>0$ divided by incident <probability current>. For incident amplitude one,
$$
\tau_D=\frac{\int_0^\infty|c|^2e^{-2\kappa x}dx}{\hbar k/m}
=\boxed{\frac{2mk}{\hbar\kappa(k^2+\kappa^2)}}.
$$
This gives a time in terms of $m,E,V_0$ through the definitions of $k,\kappa$. A simpler dimensional estimate is the penetration length divided by the incident speed, $\Delta x/v_{\rm in}=m/(\hbar k\kappa)=\hbar/[2\sqrt{E(V_0-E)}]$. The dwell time equals $(2E/V_0)$ times that estimate. Different operational timing definitions need not agree; treating the imaginary under-barrier momentum as a real particle velocity would not justify a unique traversal time.
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