It's mostly for sealed long-since discontinued DVDs, whose content cannot be found in decent quality at the usual places (AEBN, AdultEmpire...) which are clocking in at such high prices.
Somewhat an aside: if you look at the quality of early Rocco scenes at Evilangel.com (online) and compare them with the quality of the same scene on the DVD, there are miles between the two (meaning the DVD looks way better). So getting some of these older scenes on DVD makes somewhat sense (but your mileage may vary, depending on studio etc.)
Another example: initially, Alexander DeVoe released his titles through Jules Jordan. Now these get re-released under his own banner, but the new DVDs are cut for time (because the JJ DVDs were double disc with > 4 hours runtime, now edited down to fit on one DVD). So the original DVDs released through JJV become pricier...
Or some DVDs from PlatinumX/RedLightDistrict (Fresh New Faces, Feeling Black, Teens Cumming of Age) also get re-released on DVD, those are not only not edited for time, they are now reformatted to 16x9 instead of the original 4x3, all the extras are gone etc. So also in these cases the original Platinum X releases become a sought after item, and prices go up.
[Link]