Another, cheaper way is to adjust the frames for example in ACR not with "exposure" adjustment but with fill light, recovery and/or brightness. You can match the sky (more or less) this way, and ignore for example a landscape, for it is not as sensitive to differences as the sky. (Note: a water surface is almost as sensitive as the sky). However, now the colors of the frames will go apart; only the grey remains grey.
Unless these controls have improved significantly from the version of ACR used in Lightroom 1.4 to Lightroom 2, then they must be used with extreme care!
The highlight recovery tool can introduce severe artifacts in brightly lit areas that aren't over-exposed, while fill light can introduce halos around edges in mid-tones.
Adjusting the curve directly by setting a different highlight delimiter and then pulling the curve or slider down may work just as well, depending on the image, of course.
