I'll toss in another budget candidate - Axiom Champlains. They are in the same big category - mfg spec claims 3667 cu in/65L. I can say they are HUGE. More info here:
http://www.axiomgear.com/bags_panniers/champlain.php
Mine arrived today so these are first impressions with NO riding time. Construction quality appears to be better than my Performance Transit Pros. Similar heavy material with what appears to be better water resistant coating. Bungee on top for external gear and compression straps. Mounting is better - large hooks (fit 10mm racks), a thick bungee (knotted in place but looks solid enough). There is a twisting rubber block that is used to lock the panneir in place. Interestingly, it looks based on 5 minuted fiddling with it that this twist lock block doesn't work well on my Axiom Odysee panniers (go figure - so much for design configuration control).
Comes with rain covers and set of replacement hardware. Price at aebike.com was quite good but I can't find it now in their online catalog (no idea what happened - out of stock?). Lasalle model is a little saner in size and shows up in the aebike catalog at $60 a pair.
I'll post pix and a better report when I get the chance.