  We use the Scalapino-White relation between the condensation energy and the
difference between the dynamical structure factor in the normal and the
superconducting states to compute the condensation energy in the spin-fermion
model. We show that at strong coupling, the extra low-frequency spectral weight
associated with the resonance peak in the dynamical structure factor in a
superconductor is compensated only at energies $\sim J$ which are much larger
than the superconducting gap $\Delta$. We argue that in this situation, the
condensation energy is large and well accounts for the data for cuprates.
