The SRX is built on a modified version of GM's Theta platform, which is also used for the Vue (and Chevy Equinox). Theta is a "Body Frame Integral" (BFI) type chassis where the rear body panels and roof are integral to the frame, which means that the two vehicle will share similar dimensions. Once you get past the frame though, all the suspension, powertrain, interior, electronics, braking and more are all different between the two.
GM has done a lot of "badge engineering" in the past (where they slap a different logo on the same exact vehicle), but the SRX is a great example of how to do platform sharing properly. Lexus does this with the ES350/Camry and Acura with the Accord/RL, since platform sharing helps keep the MSRP low and the production streamlined.
$20 to $25k? I don't think the difference could be that high, since most comparing the Equinox to SRX would choose a loaded LTZ model Equinox. (This from a buyer who chose the 2008 Tahoe LTZ over an Escalade...because of the $20k price difference. )
A stripped Equinox or GMC version is about $24,500,,,,a loaded SRX is $44 to $50,,so yes about 20 to 25K more,,,,or TWICE as much,,,but the basic E. Or T has the little 4 cylinder, and NO options at all(except a Jack and spare tire)!,
If I compare a loaded Equinox without NAV with SRX with luxury package, the difference is about 10k in Canada. I will say SRX is well worthy the extra money. Better AWD system, better style, better material, better interior, better moonroof, better brand. The only pro of Equinox is the back seat space.